290 Mr. E. P. Adams on the Electromagnetic 



to the negative pole. On charging them while at rest a very 

 small deflexion was observed. This was not due to direct 

 electrostatic effect, but to the rush of current flowing in to 

 charge the spheres. It is only an instantaneous effect, the 

 needle coming back to its original position o£ equilibrium 

 almost immediately. The deflexion was entirely gotten rid 

 of by inserting a large water- resistance in series with the 

 battery. With this resistance in, and the spheres at rest, no 

 effect could be observed on the needle when the spheres w T ere 

 charged, or when the charge was reversed in sign. 



When the spheres were set revolving, and the electrifi- 

 cation was reversed, a distinct deflexion of the needle was 

 produced. It was difficult always to get satisfactory readings 

 of this deflexion, due to slight changes in the speed and the 

 consequent change of the zero-point. But the qualitative 

 effect was unmistakable. The deflexions were in the direction 

 to be expected; that is, a positively-charged sphere gives rise 

 to a magnetic force in the same sense as a current flowing in 

 the direction of motion. 



At first sight, there seems to be a possible alternative 

 explanation of this effect. It may be that the charges are 

 continually swept off from the rapidly-moving spheres by 

 their motion through the air, and that thus a continual flow 

 of electricity is produced in the wires which connect the 

 battery to the spheres. But the strength of such a current, 

 if it existed, would be far too small to produce the observed 

 effects, owing to the very large resistance of the water in 

 series with the battery. With this consideration, and the 

 absolute regularity with w 7 hich the effect was observed, there 

 appears to be no reasonable doubt that this is an actual 

 magnetic effect due to moving charges. 



All observations were made between the hours of 1 and 5 

 in the morning. It was impossible to get any satisfactory 

 readings in the day-time, owing mainly to the magnetic 

 disturbances produced by the electric cars. Furthermore, 

 the speed of the motor was more constant at that time 

 than during the day, since the load on the mains of the 

 power-plant varied less. It was found most satisfactory to 

 have the sensitiveness of the needle such that deflexions of 

 5-15 millimetres were produced on reversing the electri- 

 fication. Much greater sensitiveness could easily have been 

 obtained, but the zero-point varied so much that readings 

 were less reliable. The speed used was about 50 revolutions 

 per second. At this speed no trouble was experienced from 

 wind or from vibrations communicated directly to the mag- 

 netic system. But when the speed became much higher, the 



