234 Dr. J. Kerr's Electro-optic 



Elementary Measurements. 



46. The only new piece introduced is a Thomson's long- 

 range electrometer. My particular instrument is the second 

 that Mr. White has constructed of the kind, and it is modelled 

 closely on the first. For a full description and a good figure 

 of this electrometer, I must refer to Sir W. Thomson's reprint 

 of Papers on Electrostatics and Magnetism. The unit of 

 scale-value of potential which I adopt is one turn of the screw; 

 the whole range of the instrument is 200 ; and the highest 

 potential ordinarily sustainable in the prime conductor of my 

 plate-machine is about 180. 



Three guarded wires are led from the prime conductor, one 

 to the first outer ball of the plate-cell (the second outer ball 

 being always connected with earth), another to the inductric 

 plate of the electrometer, and the third to the knob of a 

 small condensing-jar, whose outer coating is connected with 

 earth. The use of the condenser is to slacken and regulate 

 the rate of variation of the potential. The pieces and con- 

 necting wires are placed properly, so as to leave the action of 

 the electrometer undisturbed. The rest of the arrangements 

 are precisely as in the experiment of extinction-bands (11), 

 and as exhibited in the diagram of 6. 



47. Method of Observation. — The plane of polarization of 

 the light rendered by the first Nicol being always at 45° to 

 the lines of force or to the horizon, and the second Mcol being 

 fixed at extinction, a weight of one or more pounds is attached 

 to one of the plates of the fixed compensator, so as to give a 

 definite initial restoration in the polariscope. Two observers 

 attach themselves, one to the electrometer and the other to the 

 polariscope; and the machine is worked at a proper rate, under 

 direction from the first observer, so as to give a slow but steady 

 rise of potential. The second observer watches for the first 

 appearance of the patch of light at the centre of the black axal 

 band (11), and he marks the instant of its appearance by a 

 sharp signal. The first observer sees whether the index of 

 the electrometer is beyond the sighted position or short of 

 it at the instant of the signal, and he works the screw ac- 

 cordingly. The observation is repeated several times if ne- 

 cessary, the screw of the electrometer being properly worked 

 each time until the index is in the sighted position at the 

 instant of the signal ; and the scale-reading of the electro- 

 meter is then recorded as the measure of the potential which 

 neutralizes the definite birefringent action introduced at start- 

 ing. Two other scale-readings are generally recorded at the 



