182 Dr. E. H. Kennarcl on 



arrangement was satisfactory, except that sudden large dis- 

 turbances were rather frequent. An earlier arrangement 

 consisting of a steel needle running in a drop of mercury 

 seemed rather better ; the substitution was made in trying 

 to locate a very troublesome disturbance, which was finally 

 traced to a minute looseness of the needle. The electrometer 

 was placed on a separate pier about 6 feet from the apparatus 

 and in line with it, the connecting wire lying approximately 

 along the axis. The sensitiveness was raised by means of a 

 concave lens from 3000 to about 18,000 mm. /volt at 4 m. 

 distance ; the spot could be read when stationary to 0'2 mm. 

 Electrometer and connecting wire were carefully screened 

 and the windings of the solenoid were connected to earth 

 through 10,000 ohms in order to render definite any residual 

 electrostatic effects. 



No means being at hand to record the instantaneous speed 

 of rotation, a device was adopted which gave a reading pro- 

 portional to the product of field-strength and speed. A 

 Faraday disk A was mounted on the end of the shaft, and a 

 galvanometer giving about 3 X 10 10 mm./amp. was connected 

 through a megohm to a brush of four brass wires bearing on 

 the bottom of the groove in the disk, and to a second brush 

 sliding opposite the earthing brush on the knob B. 



§ 3. Method of Observation. 



Since setting the condenser in rotation caused a deflexion of 

 several centimetres, readings were taken only by varying the 

 magnetic field at steady speed. Let Q ll be the charge held 

 on the inner cylinder of the condenser by electromagnetic 

 induction, and let Q = total charge on inner cylinder, con- 

 necting wire, and electrometer, = capacity of this system, 

 and V = potential of electrometer and connecting wire. 

 Then 



If, now, the magnetic field is altered, Qxis altered ; and if the 

 system is insulated a deflexion of the electrometer will result 

 corresponding to a change in its potential of 



V=-^AQ 1 . 



The usual method was to set the apparatus in rotation and 

 insulate the electrometer ; then read the spot and throw on 



