164 Dr. J. Kerr on Measurements 



optical effect respectively, I use a Thomson's Long-Bange 

 Electrometer and a Jamin's Compensator. These admirable 

 instruments are now so widely known that any description of 

 them here would be superfluous. Besides, they are described 

 fully by the inventors themselves in books which are generally 

 accessible*. 



8. Arrangements and MetJiod. — The diagram shows all the 

 optical pieces, in horizontal section through the observer's 

 eye, Q. Several of the pieces are placed exactly as in the first 

 experiment (4). These are : — the reflector, B, in the window- 



JnL 



RM 



w 



r"7 



N 



shutter; the polarizing Nicol, M; the charged cell, P; and the 

 ocular Nicol, N. The Nicols are fixed, here as formerly, with 

 their principal sections at 90° to each other, and at 45° to the 

 vertical. After the cell, and close to it, comes the compen- 

 sator C, which is placed with great care and very stably, with 

 the optic axes of its quartz prisms properly directed, one ver- 

 tical and the other therefore horizontal, and both perpendi- 

 cular to the ray. The narrow beam transmitted by the com- 

 pensator, and received at Q, passes of course through the 

 central parts of the electro-optic field. After the compensator 

 comes a convergent lens D, which' is fixed by trial in such a 

 position as to give the observer at Q a distinct view of the 

 black band and reference-wires in the compensator. Some 

 changes were made afterwards in the optical arrangements ; 

 but I will describe these in the proper place. 



Two wires are led permanently from the conductors within 

 the cell — from the lower to earth, and from the upper to prime 

 conductor. Wires are also led permanently from prime con- 

 ductor to inductric or upper plate of the electrometer, and to 

 the inner coating of a good Leyden jar whose outer coating is 

 connected permanently with earth. 



The index of the compensator being at the zero-point, the 

 observer at Q sees a strong black band exactly midway between 

 the two parallel reference-wires. When the machine is set in 



* For the Electrometer, see Sir W. Thomson's Eeprint of Papers on 

 Electrostatics and Magnetism, § 383; and for the Compensator, see Jamin's 

 Cours de Physique, tome iii. pp. 622, 639. 



