ELECTROMETER

ELECTROMETER. In various parts of the article ELECTRICITY, we have described a great variety of instruments for ascertaining the presence of electricity, and measuring its quantity or proportion.

But there are several instruments of this kind that have not been described in that article; and as they are well deserving a place in this work, either from the ingenuity of their construction, the reputation of their inventors, or the intrinsic value of the instruments themselves, we shall give an account of them here.

Fig. 1. Plate CC. is a geometrical representation of Mr Cavallo's improved atmospherical electrometer, of half its real size. The principal part of this instrument is a glass tube CDMN, cemented at the bottom into the wooden piece AB, by which part the instrument is to be held when used for the atmosphere; and it also serves to screw the instrument into its wooden case ABO, fig. 2. when it is not to be used. The upper part of the tube CDMN, is shaped tapering to a smaller extremity, which is entirely covered with sealing-wax, melted by heat, and not dissolved in spirits. Into this tapering part a small tube is cemented, the lower extremity G of which being also covered with sealing-wax, projects a short way within the tube CDMN. Into this smaller tube a wire is cemented, which with its lower extremity touches the flat piece of ivory H, fastened to the tube by means of cork; the upper extremity of the wire projects about a quarter of an inch above the tube, and screws into the brass cap EF, which cap is open at the bottom, and serves to defend the waxed part of the instrument from the rain, &c. In fig. 3. a section of this brass cap is represented, in order to show its internal shape, and the manner in which it is screwed to the wire, projecting above the tube L. The small tube L, and the upper extremity of the large tube CDMN, appear like one continued piece, on account of the sealing-wax, which covers them both. The conical corks P of this electrometer, which by their repulsion show the electricity, &c. are as small as

can conveniently be made, and they are suspended by exceedingly fine silver wires. These wires are shaped in a ring at the top, by which they hang very loosely on the flat piece of ivory H, which has two holes for that purpose. By this method of suspension, which is applicable to every sort of electrometer, the friction is lessened almost to nothing, and thence the instrument is sensible of a very small degree of electricity. IM, and KN, are two narrow slips of tin-foil, stuck to the inside of the glass CDMN, and communicating with the wooden bottom AB; they serve to convey off that electricity, which, when the corks touch the glass, is communicated to it, and being accumulated, might disturb the free motion of the corks.

In regard to its use, this instrument may serve to observe the artificial, as well as the atmospherical electricity. When it is to be used for artificial electricity, this electrometer is set upon a table or other convenient support; then it is electrified by touching the brass cap EF with an electrified body, which electricity will sometimes be preserved for more than an hour. Mr Cavallo had one of these electrometers which would remain electrical for more than twelve hours, though in a room without a fire. If in an electrified state, any electrified substance be brought near the cap EF, the corks of the electrometer, by their converging, or by increasing their divergency, will shew the species of that body's electricity.

It is necessary to remark, that to communicate any electricity to this electrometer, by means of an excited electric, e. g. a piece of sealing-wax, (which we suppose is always negatively electrified), is not very readily done in the usual manner, on account of the cap EF being well rounded, and free from points or sharp edges. By the approach of the wax, the electrometer will be caused to diverge; but as soon as the wax is removed, the wires immediately collapse. The best method to electrify it, is to bring the excited wax so near the cap, that one or both the corks may touch the side of the bottle CDMN; after which, they will soon collapse and appear unelectrified: if now the wax be moved, they will again diverge, and remain electrified positively.

When this instrument is to be used to try the electricity of the fogs, the air, the clouds, &c. the observer is to do nothing more than to unscrew it from its case, and, holding it by the bottom AB, to present it to the

A

open

Electrometer—open air, a little above his head, so that he may conveniently see the corks P, which will immediately diverge if there be any sufficient quantity of electricity; whose nature, i. e. whether positive or negative, may be ascertained by bringing an excited piece of sealing-wax, or other electric, towards the brass cap EF.

It is perhaps unnecessary to remark, that this observation must be made in an open place, as the roads out of town, the fields, the top of a house, &c.

The principal advantages of this electrometer, as stated by Mr Cavallo, are as follows.

1. The smallness of its size. Mr Cavallo made one so small, that its case, which was of brass, measured only three inches and a half in length, and nine-tenths of an inch in diameter, and yet it acted perfectly well.

2. Its being always ready for experiments, without fear of entangling the threads, or having an equivocal result by the sluggishness of its motion.

3. Its not being disturbed by wind or rain.

4. Its great sensibility; and,

5. Its keeping the communicated electricity longer than any other electrometer.

II. Saussure's ELECTROMETER. M. de Saussure's electrometer, with which he made the observations on atmospheric electricity, that have been related in the second chapter of Part V. of the article ELECTRICITY, and represented at fig. 4. is much the same with that of Mr Cavallo above described. The following are the most material circumstances in which they differ: First, the fine wires, by which the balls are suspended, should not be long enough to reach the tin-foil which is pasted on the inside of the glass, because the electricity, when strong, will cause them to touch this tin-foil twice consecutively, and thus deprive them in a moment of their electricity. To prevent this defect, and yet give them a sufficient degree of motion, it is necessary to use larger glasses than those that are generally applied to Mr Cavallo's electrometer; two or three inches in diameter will be found to answer the purpose very well. But as it is necessary to carry off the electricity which may be communicated to the inside of the glass, and thus be confounded with that which belongs to those substances that are under examination; four pieces of tin-foil should be pasted on the inside of the glass; the balls should not be more than one-twentieth of an inch diameter, suspended by silver wires, moving freely in holes nicely rounded. The bottom of the electrometer should be of metal; for this renders it more easy to deprive it of any acquired electricity, by touching the bottom and top at the same time.

In order to collect a great quantity of electricity from the air, the electrometer is furnished with a pointed wire, 15 inches or two feet long, which unscrews in three or four pieces, to render the instrument more portable; see fig. 4. When it rains or snows, the small parapluie, fig. 5. is to be screwed on the top of the instrument, as by this its insulation is preserved, notwithstanding the rain.

This instrument indicates not only the electricity of fogs, but that also of serene weather, and enables us to discover the kind of electricity which reigns in the atmosphere; and to a certain degree to form an estimate of its quantity, and that under two different points of

view, the degree of intensity, and the distance from Electrometer—the earth at which it first begins to be sensible.

A conductor raised for the purpose of making observation on atmospheric electricity will be found to exhibit signs of electricity, only when the electric fluid is more or less condensed in the air, than in the earth. Though the air resists the passage of the electric fluid, it is not absolutely impermeable to it; it suffers it to pass gradually, and generally with more ease in proportion as its mass or thickness is less. It is therefore interesting to discover at what height it is necessary to be elevated, in order to find a sensible difference between the electricity of the earth, and that of the air. A very sensible difference may be generally discovered by this instrument, at the distance of four or five feet from the ground; sometimes it may be seen if the instrument is placed even on the ground; while at others, it must be raised seven or more feet before the balls will open; sometimes, though seldom, this height is not sufficient. This distance is generally greatest when the electricity is strongest, though necessarily modified by a variety of circumstances, some of which are known, as the degree of dryness or humidity of the air, and others are unknown.

The degree of intensity, at a given height, may be discovered thus; raise the electrometer, and judge by the divisions which are placed on the edge of it, the degree of their divergence. To find the relation between this degree of divergence, and the force of the electricity, M. de Saussure took the following method: As he could not with certainty double or triple a given quantity of electricity; yet as a given force may be reduced one half, a fourth or eighth, &c. by dividing between two equal and similar bodies, the electricity contained in one; he took two of his unarmed electrometers, which were as similar as possible, and electrified one of them, so that the balls separated precisely six lines; he then touched the top thereof by the top of that which was not electrified; in an instant the electricity was equally divided between them, as was evident by the divergence of the balls, which was four lines in each; consequently a diminution of half the density had only lessened the divergence one-third. One of these electrometers was then deprived of its electricity, and was afterwards brought in contact with the other, as before; the remaining electricity divided itself again between them, and the balls fell from four to twenty-eight lines, nearly in the same proportion as before; in the third operation they fell to nineteen; in the fourth to one, where he was obliged to stop, as there was not now sufficient force in the fluid to pass from one electrometer to the other, and distribute itself uniformly between them. The same experiment, repeated several times, gave very nearly the same results. Negative electricity decreased also in the same proportion as the positive. The following table may therefore be considered as giving a general, though not exact idea of the increase in force, which corresponds to different degrees of divergence in the balls; it is only calculated to every fourth of a line; the force of electricity is always expressed by whole numbers, as it would be ridiculous to put a greater degree of exactness in the numbers than is to be found in the experiments which form the bases of the calculation.

Those who are desirous to carry this measure of the electric force further, may do it by having similar electrometers constructed, but made upon a larger scale, and with heavier balls, which would only separate one line, with the degree of electricity that makes the smaller ones diverge six lines; these would consequently measure a force 1024 times greater than that which forms the unity of the preceding table; and thus by degrees we may be enabled to discover the ratio of the strongest discharge of a great battery, or perhaps even of thunder itself, to that of a piece of amber, which only attracts a bit of straw or any other light substance.

In order to observe the electricity of the atmosphere with this instrument, we must first bring the electric fluid contained in the electrometer to the same degree of density with that at the surface of the earth; this is easily done by letting the bottom and top touch the ground at the same time; then raise the point, keeping the bottom still in contact with the ground, from whence it may be lifted up in a vertical position till the balls are level with the eye.

The second circumstance is to render the divergence of the balls, which is occasioned by the electricity of the air, permanent. This is effected by touching the top of the electrometer with the finger; but here the acquired electricity becomes contrary to that of the body by which they are electrified. Let us suppose, for example, that the electrometer is at five feet from the ground, and the balls diverging; touch the top of the electrometer with the finger, and the balls will close; but they will again open, if the electrometer is withdrawn from the influence of the electricity of the air, by being brought nearer the ground, or into the house. M. Saussure only employed this method when the electricity was so weak that he could not perceive any until the electrometer was raised considerably above his eye: as in this case he could not perceive the diver-

gence of the balls, he always endeavoured to obtain a permanent electricity in the foregoing manner.

The following example will render the use of the foregoing observations more familiar. Choose an open situation free from trees and houses, screw the conductor on the top of the electrometer, lay hold of it by its base, and place it so that the base and conductor may touch the ground at the same time; then elevate it to the height of the eye, and observe the quantity of lines, or fourths of a line, that the balls have diverged; now lower it till the balls almost touch each other, and observe at what distance the top of the conductor is from the ground; and this is the height from the ground at which the electricity of the air begins to be sensible. If the electricity of the air is sufficiently strong to make the balls diverge when it stands upon the ground, one of the lengths of the electrometer must be unscrewed from it. If the balls, however, still diverge, the other parts of the conductor should also be unscrewed, and you may mark down, that the electricity is sensible at zero, or on the surface of the earth. If, on the contrary, the electricity is so weak, as not to cause the balls to diverge when they are even with the eye, and consequently when the conductor is two feet higher, or seven feet from the ground, you should then raise it a foot higher; while it is thus elevated, touch the top with the other hand; when this hand is taken away, lower the electrometer, and if it is electrified, you may say the electricity is sensible at eight feet; if it is not, raise it as high as the arm can reach, and repeat the same operation; if any electricity is found, write down electricity sensible at nine feet; if not, mark 0, or no electricity relative to this instrument, and this mode of employing it; for signs of electricity may still be obtained, by throwing a metallic ball 50 or 60 feet into the air, which is at the same time connected with the electrometer by a metallic thread.

One advantage of this instrument is, that it will often exhibit signs of electricity when none can be obtained from a conductor of 100 feet in height, because it can more easily be preserved from humidity, &c. which will destroy the insulation of the large conductors.

This electrometer may be used instead of the condenser of M. Volta, by only placing it on a piece of oiled silk, somewhat larger than the base of the instrument; but in this case, it is the base, and not the top of the instrument, which must be brought into contact with the substance whose electricity is to be explored.

It is easy to discover also by this instrument, the electricity of any substance, as of cloths, hair of different animals, &c. For this purpose, it must be held by the base, and the substance rubbed briskly (only once) by the ball of the electrometer; the kind of electricity may be ascertained in the usual manner. It is proper, however, to observe here, that as the top of the electrometer acts in this case as an insulated rubber, the electricity it acquires is always contrary to that of the rubbed body.

III. Cadet's ELECTROMETER, is thus described by the author, as translated in Nicholson's Journal.

Fig. 6. In a glass tube A, 18 or 20 inches long, Fig. 6. is inclosed another shorter tube X, sealed at both ends. This tube contains a graduated scale: one of the ends of these two tubes is cemented in a handle of turned wood,

Electrometer-wood, C, by which it is held in the hand; the other end is closed by a brass cap, D; the distance between the extremities of the small tube and that of the large one is filled with red wax, B, B; on the cap D is screwed at pleasure, either a ring E, or a brass hook F. The ring is used for applying the instrument to the ball of a conductor, and the hook when it is hung to a ring: on the cap D is a brass stem G, terminating by a knob. This stem is bended, and the extremity of its knob must be directly beneath the line with which the graduated scale of the small tube commences.

Round the large tube is a brass ring H, half of which extends to the length of twelve or fifteen lines in the form of a half tube P, applied against the sides of the tube. This gutter serves to mark the degrees, by sliding along the graduated scale by means of a button beneath I. On the ring H is fixed one of the small electrometers invented by Saussure, K, K, which is surmounted by a stem V, on which stem is fixed at pleasure either a point L, or a ball M, of the same size as that which terminates the stem G, opposite which it is placed. The extremity of this point or ball must be placed immediately over the extremity of the half tube or scale P, and horizontally to the centre of the ball, which terminates the stem G.

At the top of Saussure's electrometer is a small ring N, which serves to connect it with the chain Z when required.

To explain the use of this instrument by a single experiment, charge a Leyden jar, till the spontaneous overflowing announces it to be saturated. Then place the ring E on the knob of this bottle, and cause the electrometer of Saussure, armed with its point, to slide towards it. Observe the degree at which the divergence of the thread stream commences, and at that instant suppress the point, and adapt in its place the ball M. Continue to advance the electrometer of Saussure till the electric pressure of the atmosphere in the jar causes the threads to diverge; again observe the degrees, replace the point L, and close the shutters of the room; then continue to advance the electrometer till the luminous point appears, which again affords new degrees. Lastly, replace the ball M, and fix the chain Z to the small ring N; cause it to communicate with the exterior coating of the jar, and advance the electrometer till the explosion takes place. Then comparing the different degrees, we may ascertain the comparative difference between the respective methods.

As soon as these relative proportions have been once accurately ascertained by attentive observations, one of those methods alone will be sufficient for measuring the intensity of electricity; and, in fact, if the body intended to be submitted to examination be little charged with the electric fluid, the diverging of the threads, by means of the point, will fix the limits of the electric atmosphere: if it be more, the pressure of the atmosphere on the ball M, which is substituted for the point, will indicate this quantity. In short, if the body be loaded with a considerable mass of electric matter, it will be shown by the luminous point. If a Leyden jar, instead of being positively, is negatively electrified, the point indicates it at the same time that it measures the electric atmosphere, for instead of a luminous point, a star

will be observed upon the ball of the jar, and another Electrometer at the end of the point.

Let us now apply this electrometer to useful observations.

In order to connect the idea of a determinate quantity of fluid to each degree of the electrometer, it is necessary to compare these degrees with the known quantities. Suppose for instance we have a jar, the coating of which is six inches square; electrify it till a spontaneous discharge takes place, and remark, by means of Henley's electrometer, at what degree this discharge is effected. Again, electrify the jar, till it is nearly saturated, and measuring with this electrometer, observe, that the luminous point appears for instance at two degrees; then say, that when the electrometer, applied to an electrified body, marks two degrees, the body contains six inches square of electricity. Repeat this experiment with a plate of glass, the coating of which is seven, eight, ten, or twelve square inches, and we may form a scale of proportion, which is of the greatest utility in accurate experiments.

"In endeavouring to ascertain some of these propositions, (says M. Cadet), I have made an observation which has convinced me of the utility of my electrometer in discovering the capacity of electric apparatus. Having taken a jar from an electric battery, I electrified it, and measured it with a point which I passed along a string of silk; on observing the distance at which the luminous point appeared, I joined this jar to another of the same size, and imagined that by doubling the quantity of matter, the measure I had taken would also be doubled; on the contrary, however, the latter measure was not more than about one-third of the former: I then added a third bottle; and still obtained nearly the same result; whence the following proposition appears to be established; namely, that the extent of the electric atmosphere is in an inverse ratio to the quantity of fluid accumulated. Another observation which I have several times made, on measuring the electric atmosphere of a conductor, is, that the limits of this atmosphere form an elliptic figure around the body, nearly similar to that represented at fig. 7.

"This doubtless arises from the electrified body suspended in a chamber, being nearer to the earth than the ceiling; but it would be a curious experiment to measure it at an equal distance from every attracting body, in order to observe whether the fluid has not really a tendency to descend towards the earth, rather than in any other direction. It is my intention to repeat this experiment, as I consider it of great importance to ascertain whether electricity gravitates towards the globe.

"From these first attempts, I conceive my electrometer would be well adapted for measuring the absolute capacity of Leyden jars, and also their capacity with regard to their size, or to the quality of the glass of which they are constructed; for the latter, by its greater or less density, absorbs a greater or less quantity of fluid."

IV. Lawson's ELECTROMETER. This is a simplified improvement on Brooke's Steelyard electrometer, and should have been described when that instrument was mentioned, instead of Mr Adams's; but it did not occur to us till after that sheet was printed.

The

Electrometer. The following account is given of this electrometer, in a letter from Mr Lawson to the editor of the Philosophical Magazine.

"Some time ago it struck me that some additions to Brooke's electrometer might be made, so as to fit it for a good discharging electrometer to measure the repulsion between two balls (of a certain size) in grains, and also effect the discharge of a battery at the same time. The instrument known by the name of Cuthbertson's discharging electrometer, (See ELECTRICITY, N^o 203.) was at that time the best, and indeed the only instrument for discharging batteries or jars by its own action, then made; but I think this will be found, in the essentials, and in the theory and use, a more perfect instrument.

Fig. 8. "On the basis (fig. 8.) is fixed the glass pillar G, supporting the hollow brass ball B. I is a light graduated brass tube, divided (from the weight W towards the ball B) into thirty parts, representing grains. W is a sliding weight. L, a light brass ball screwed to the end of the tube I. On the other end of which tube adjusts the heavy counterbalance ball C, the tube I and its two balls being suspended at their common centre of gravity by a silk line in the centre of the ball B, the mechanism of which is shown in fig. 9. The brass ball F is stationary, and of the same size as the ball L; and is fixed by, and adjusts close to, the ball L, or at any lower station between that and the ring r. The brass tube to which the ball A is fixed is divided into inches, halves, and quarters: (a more minute division is unnecessary and improper). The divisions begin, or the line o is marked on the said tube at the ring r, when the three balls A, L, F, are close together. The ring r serves as an index, as the divisions pass in succession into the glass tube P on lowering the ball A. The hook H is screwed into the base of P. The quadrant, or Henley's electrometer, Q, is supported in a long brass stem, to keep it out of the atmosphere of the lower part of the instrument. Fig. 9. shows the internal construction of the ball B, fig. 8. In the first place the ball screws in half, horizontally. The light tube I passes through the ball, and is suspended nearly in the centre of it by some silk twist, s, which small silk twist is fixed into the eye of the adjusting wire, a, part of which wire is filed square and goes through the square hole k. The nut n screws on a, and serves to adjust the light tube I vertically. The light plates PP are of copper, and move freely on the wire w somewhat like a hinge, and rest on the top wires CC, serving to make the direct communication between the inside and out of the battery or jar. NN are notches serving to let the tube I descend when the discharge is made. Into the tube Z the glass pillar is ground. Note, that at the bottom of the notch N is a piece of brass filled with a Y, and so placed as to keep the centres of the balls L and F, fig. 8. under each other when they come close together.

"When the instrument is adjusted, which is done by placing the weight W, fig. 8. at o on the line of grains, and then screwing or unscrewing the counterbalance ball C, till the tube I rises slowly into its horizontal position; then set the ball A at the distance from the ball L that you choose, and the weight W placed at the division or number of grains that you wish the repulsive power of the electricity to arrive at before the discharge

is made: this being done, connect the battery or jar Electrometer. with the ball B, by means of the wire y, the end of which goes into B at the hole X, and should stand at right angles to B, the ball of y resting on the battery: then connect the outside of the battery or jar with the hook H. As the battery charges, the electrometer Q continues to rise; and when it is so highly charged that the repulsive power between the balls L and F is equal to the number of grains at which the weight W was placed, the ball L will descend, and deliver the charge of the battery to the ball A. The substance or thing through which the shock is intended to be passed, must form part of the communication between the hook H and the outside of the battery or jar."