86 Prof. J. J. Thomson. On the Propagation of the [Jan. l.'j. 



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faces, and the effect of very different terminals on the velocity and 

 direction of the discharge readily investigated. The bulbs in these 

 vertical tubes were also found very useful as receptacles for sulphuric 

 acid for drying the gas left in the tube. 



The revolving mirror was driven at a speed varying from 400 to 

 500 per second by a Gramme machine through which the current 

 from twelve large storage cells was sent. The mirror first used was 

 mounted on ball bearings such as are used for bicycles; the axle had, 

 however, too much play, and it was eventually discarded for one made 

 by the Cambridge Scientific Instrument Company, and designed by 

 Mr. Bartlett, the assistant at the Cavendish Laboratory. It is repre- 

 sented full size in tig. 2. The spindle carrying the mirror M runs on 

 parallel bearings in the uprights A, A. The ends of the spindle are 

 rounded, hardened, and polished, and are in line with but just do not 



