Moving Charged Spheres. 159 



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 to 

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

 small deflection 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 of equilibrium 

 almost immediately. This deflection 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 were charged, or 

 when the charge was reversed in sign. 



When the spheres were set revolving, and the electrification 

 was reversed, a distinct deflection of the needle was produced. 

 It was difficult always to get satisfactory readings of this 

 deflection due to slight changes in the speed and the conse- 

 quent change of the zero-point. But the qualitative effect was 

 unmistakable. The deflection was 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 explana- 

 tion 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 which 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 one and 

 five in the morning. It was impossible to get any satisfactory 

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

 turbances 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 a deflection of 5-15 millimeters was 

 produced on reversing the electrification. Much greater sen- 

 sitiveness could easily have been obtained, but the zero-point 

 varied so much that the 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 commu- 



