ELECTROMETERS. 225 



and was itself the cause of the electric force. Faraday 

 preferred to employ the instrument thus. He gave a per- 

 manent charge to the torsion ball and placed it in a definite 

 position, suppose 30 from the zero mark on the scale pasted 

 round the cylindrical case of the instrument. He then 

 introduced the carrier ball or proof plane, bearing its 

 charge, which of course displaced the torsion ball by attrac- 

 tion or repulsion. Turning the button of the torsion head 

 the torsion ball was brought back to its first position, and 

 the torsion required in order to do this was determined. 



In his celebrated Experimental Researches on Electricity, 

 Faraday describes minutely the precautions that must be 

 taken in using the torsion balance. He considered it a very 

 valuable instrument in good hands : and in his hands it 

 proved indeed a most valuable instrument. It was by 

 means of it that all his grand series of electrostatic re- 

 searches was carried on. 



From the torsion balance of Coulomb there came a 

 variety of other instruments more or less like it. Here for 

 example is Peltier's electrometer, which if it had protection 

 of the movable parts would be an excellent instrument. 

 Instead of an arm supported on a torsion fibre we have here 

 a long aluminium needle pivotted on a fine needlepoint, and 

 to the movable needle there is attached a very short magnet. 

 The small magnet gives the directive force instead of the 

 torsion of the fibre. To use the instrument it is set with 

 the magnet in the magnetic meridian and the two movable 

 arms in contact with these two repulsion balls which take 

 the place of the carrier ball in the torsion balance. When 

 a charge is given to the instrument by means of this 

 charging rod connected with the repulsion balls, the charge 

 is also communicated to the movable needle, and repulsion 

 is the result. The electric repulsion is balanced by the 

 tendency of the magnet to return to its normal position in 

 the magnetic meridian. 



Here again is a similar instrument by Kohlrausch. It 

 consists also of a conductor, a'portion of which is a magnet, 

 and repulsion plates similar to the two repulsion balls which 

 were used in the Peltier electrometer. Kohlrausch calls 

 this instrument a sine electrometer because in his way of 

 using it the forces are proportional to the sines of the angles 

 of deflection of the movable needle. 



