charg elec- 118 consult the account given by Mascart, Traite d Elcctr kite Statiquc, lorn. i. 313-316, and Eiess, I.e. The torsion balance of Coulomb is another instrument s the direct measurement of electrical quantity. For its construction and use see the article ELECTRICITY, p. 18, The discharging electroscope of Gaugain belongs to the present class of instruments. It consists (fig. 2) of an ordinary (old- icope. fashioned) gold-leaf electroscope, with the addi tion of a small knob B, connected with the metal sole of the instrument, and standing a little to one side of one of the leaves. The charge on any conductor is measured by connecting it with the knob A through a sufficient length of wet cotton to retard the discharge properly. When a certain amount of electricity has reached the gold leaf, it is attracted to the knob B and is discharged; it then falls back, is recharged, then discharged by contact with B a second time, and so on. It is lound that the same quantity of electricity is discharged at each contact if the process be properly regulated; so that the whole FIG. 2. Dischaig- charge on the conductor is measured by the ing Electroscope" number of oscillations of the gold leaf required to discharge it completely. l The rest of the instruments (save one) to be described may be classified under the three beads given by Sir Wm. Thomson in his valuable report on electrometers, 2 viz., (1) repulsion electrometers, (2) attracted disc electrometers, and (3) symmetrical electrometers. I. Repulsion Electrometers. The electroscopic needle of Gilbert is the oldest specimen of a repulsion electroscope. The linen threads of Franklin, and the double pendulum used by Canton, Du Fay, and others, which was an improvement thereon, are typical of another species of electroscope coming under the same genus. ") embodies the double pendulum vallo s Cavallo s electroscope 3 (fi ctro- principle. It consists of pe A two fine silver wires loaded with small pieces of cork or pith, and suspended inside a small glass cylin der. Through the cap which closes the cylinder passes the stout wire frou? which the pendulums are suspended. This wire ends in a thimble-shaped dome A, which comes down very nearly to the cap; the out side of the cap and part of the wire are covered with sealing wax, and the object of the dome is to keep moisture from the stem, so that the electroscope could be used in the open air even in rainy weather. ^Cavallo s Electroscope. To add to the sensitive ness of the instrument two strips of tinfoil are pasted on the glass at B and C opposite the pith balls. An electroscope similar to this was used by Saussure. 4 Volta used a pair of straws instead of the pith ball pendulums. snnet s By f ar the most P er f ect f rm f electroscope on the double ild-leaf pendulum principle is the gold-leaf electroscope of Bennet. 5 Fig. ectro- 4 represents a modern form of this instrument. The gold leaves are gummed on the two sides of a flat piece of metal carried by a stout stem, which passes through the top of a glass shade and ends in a flat disc. By means of this disc we may convert the instru ment into Volta s condensing electroscope (already described, see ELECTRICITY, p. 34). Inside the glass shade, and rising well over the leaves, stands a cylinder of wire gauze, which ought to be in metallic connection with the earth, or with some conductor whose potential is taken as the standard of reference. The introduction of the wire cylinder is due to Faraday, and is an essential improve ment; it is absolutely necessary, in fact, to convert the instrument into a trustworthy indicator of differences of potential. It serves the double purpose of protecting the leaves from external disturbing influences, and of ensuring that the instrument always indicates the difference between the potential of the body connected with the ope. 1 There is a correction for residue, see Mascart, t. i. 317, &c. 2 Brit. Assoc. Rep. 1867, or Reprint of Papers on Electrostatics and Magnetism, 343. 3 1777? 4 Riess, 49 and 50. 5 Phil. Trans., 1787. eaves and another definite potential. Thus, if we insulate the sole of the electroscope, and connect A with the leaves, and B with the [jauze, the divergence of the leaves corresponds to the difference Between the potentials of A and B, and will always be same for
- he same potential difference. 8 Hence, if the divergence of the
eaves were read off by means of a properly constructed scale, the nstrument might be used as a rough electrometer. The value of FIG. 4. Bennet a Electroscope. FIG. 5. Henley s Electrometer. the graduation would of course have to be determined by experi ment. Peclet did, as a matter of fact, use the gold-leaf electroscope in this way. The electrometer of Henley, 7 sometimes called Henley s quadrant Henley I electrometer (fig. 5), may be" taken as the type of single pendulum electro- electroscopes. It consists essentially of a pendulum A hinged to meter. a vertical support C, which carries a vertical graduated semicircle B, by means of which the deviation of A from the vertical can be read off. This form of electroscope is, or was, much used foi indicating the state of electrification, of the prime conductors of electric machines. The stem is screwed into the conductor, and the divergence of the pendulum indicates roughly the charge. The sine electrometer of August, represented in fig. 6, is a modi- Pendu- fication of the single pendulum electroscope, analogous in principle him siu< to Pouillet s sine compass. A is a pendulum suspended by two threads to secure motion in one plane; B is a ball fixed to the case, and connected with a suitable electrode. Any charge is given to A; B is charged with q units of electricity; the case is turned through an angle * in a vertical plane until the distance between A and B is the same as it was when both were neutral; then, if the charge on A be always the same, q cc sin < , This instrument is interesting on ac count of the principle employed in its construction; but we are not aware that it has ever been used in practice. g _ gine Electrometer . Another class of instruments, in which the movable part is a horizontal arm turning about a vertical axis, may be looked upon as the descendants of Gilbert s electro scopic needle. The electrometer of Peltier and its modification into a sine electrometer (by Riess) are instruments of this class. Descriptions of both will be found in Mascart, 291 uud 292. _ Dellmann s electrometer (fig. 7) is constructed on a principle Dell- similar to that applied in the two instruments last named. D is a mann. needle, formed of light silver wire, suspended by a fine glass fibre from a torsion head A. Below the needle is a piece of sheet metal NE, divided half through by a notch in the middle, and then bent in opposite directions on both sides of the notch, so that, when looked at end on, it appears like a Y. Underneath NE is a 8 It was by no means safe to take this for certain in the old instru ments, owing to the electrification of the glass.
7 Phil. Trans., 1772.