28 J. L. Moore — Electrical Discharge from the point 



As the secondary of the induction coil is short circuited twice 

 during each rotation of the two-part commutator, the potential- 

 difference at the terminals of the secondary must be zero at the 

 instant of short circuit, to prevent serious sparking at the con- 

 tact brushes. This adjustment is easily made, and serves as a 

 delicate means to determine when one of the electrodes of a 

 tube, connected to the terminals of the commutator, is at every 

 instant, throughout the waves of potential, positive, and the 

 other electrode, at every instant, negative. 



The tube, being connected to the mercury pump, is levelled 

 and otherwise adjusted until the radiometer vanes hang cen- 

 trally in their containing vessels. The whole tube is then 

 thoroughly exhausted and dried, (the glass stopcocks C, C, fig. 

 1, being open). A separate light source of constant illumination 

 is provided for each radiometer. Incandescent lamps have 

 been found most desirable for this purpose. An image of the 

 tiliment of the incandescent lamp, as reflected from the con- 

 cave mirror on the radiometer vane, is received on a white 

 graduated scale, located between the radiometer and the light 

 source. The pressure of the residual gas in the tube is then 

 adjusted, by means of the mercury pump, until any further 

 reduction of the pressure decreases the deflection of the 

 radiometer vane. One can in this way be sure that the pres- 

 sure in the tube corresponds to some point on the descending 

 branch of the sensibility curve. 



The tube is then left connected to the mercury pump, 

 and the uni-directional potential-difference, obtained as de- 

 scribed, is connected to the electrodes (a) and (b), tig. 1, — (a) 

 being made negative, (b) positive. At the pressure above in- 

 dicated (about 10. 10~ 6 Ats.), a clean cut discharge stream is 

 seen to go out from the negative electrode (#), pass continu- 

 ously through the opening in the funnel-shaped diaphragm (D), 

 strike the positive electrode (b), and cause fluorescence on the 

 glass walls of the tube back of (b), not shielded by the elec- 

 trode. The glass stopcocks (C, C) are now closed, the induction 

 coil and the rotating commutator are thrown out of action. 

 The images of the incandescent lamp Aliments on the grad- 

 uated scales are allowed to come to rest. The stopcock of the 

 radiometer on the B-side of the diaphragm (D) is then care- 

 fully opened. The deflection of the radiometer, as indicated 

 by the reflected image of the incandescent lamp filiment, is 

 seen permanently to decrease. The change in deflection on 

 opening the cock amounted in some experiments to as much 

 as 10 cra when the scale was 75 cm from the radiometer. On 

 opening the cock connecting the radiometer, on' the A-side of 

 the diaphragm, with the tube, very little change of deflection 

 is noted. This is readily understood when the relative 



