158 Adams — Electromagnetic Effects of 



vibrations. By means of control magnets placed on the brass 

 plate M any required degree of sensitiveness can be given to 

 the needle-system. All metallic parts of the magnetometer 

 tube and supports are earthed. A single turn of wire at K 

 serves to determine the needle-constant. The deflections are 

 read by means of the telescope and scale T, placed at a dis- 

 tance of 3 meters from the mirror. 



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

 power motor, at a distance of 7 meters from the magnetome- 

 ter. 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, FF, 

 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 AA is at a distance of 1 

 meter 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 

 battery of 10,000 storage cells used by Professor Trowbridge 

 in investigations in spectrum analysis,- etc., and 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 sur- 

 rounding walls, except for a short distance, where they are 

 carried in thick-walled glass tubes. A commutator is inserted 

 to reverse the sign of the charges of the spheres. 



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

 duced 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 impossi- 

 ble to take readings owing to the continual vibration 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 bot- 

 tom by a very thin glass plate, which the spheres just clear 

 when revolving. 



Not the slightest movement of the needle could be observed 

 when the motor alone was run, or when the motor and steel- 

 shaft along the floor were run. But the cutting of the earth's 

 magnetism by the brass axle was sufficient to produce a deflec- 

 tion of several centimeters. As long as the speed remained 

 perfectly constant this gave no trouble. But if the speed 



