Effects of Moving Charged Spheres. 289 



Power for driving the spheres is furnished by a 4 horse- 

 power motor, at a distance of 7 metres from the magneto- 

 meter. A heavy iron casting L, in front of the motor, gives 

 additional screening of magnetic disturbances due to the 

 motor. The motor is belted to the steel countershaft, F F, 

 which turns in hangers placed along the cement floor. A 

 rigid wooden framework is built up from the floor to carry 

 the revolving spheres. The axle A A is at a distance of 

 1 metre from the floor. The brick piers which support the 

 magnetometer are entirely separate from the floor and from 

 the framework which carries the spheres. 



The electricity for charging the spheres is furnished by 

 the storage-battery of 10,000 cells used by Professor Trow- 

 bridge in investigations in spectrum analysis &c, which has 

 been described by him in this journal. The battery is on 

 the third floor of the laboratory, while the apparatus for this 

 experiment was set up in the basement. The wires leading- 

 down from the battery are well separated from each other 

 and from surrounding walls, except for a short distance, 

 where they are carried in thick-walled glass tubes. A com- 

 mutator is inserted to reverse the sign of the charges of the 

 spheres. 



A good deal of difficulty was met with due to the wind 

 produced by the revolving spheres. These have a velocity of 

 about three miles a minute, and the wind was sufficient to 

 shake the shelf supporting the magnetometer so that it was 

 impossible to take readings owing to the continual vibra- 

 tions of the needle-system. This made it necessary to build 

 a shield around the spheres, to keep the wind from blowing 

 directly on any part of the magnetometer support. In order 

 to bring the needles as close as possible to the spheres, the 

 top board of the shield has a hole cut in it into which the 

 lower end of the magnetometer-tube is placed. The hole is 

 closed on the bottom by a very thin glass plate which the 

 spheres just clear when revolving. 



Not the slightest movement of the needle could be ob- 

 served when the motor alone was run, or when the motor and 

 steel countershaft along the floor were run. But the cutting 

 of the earth's magnetism by the brass axle and spheres was 

 sufficient to produce a deflexion of several centimetres. As 

 long as the speed remained perfectly constant, this gave no 

 trouble. But if the speed varied by even a small amount, 

 this was very troublesome, and was one of the principal 

 sources of error in the experiment. 



The two sets of spheres are charged oppositely, one being 

 connected to the positive pole of the battery, and the other 



