376 Mr. John Trowbridge on 



poles or poles of the same name could be opposed. Two 

 small coils of fine wire were also slipped npon the ring ; each 

 of these small coils was connected with a telephone. The 

 diaphragm of one telephone gave a horizontal line of light, 

 and the diaphragm of the other a vertical line, the combina- 

 tion of the two amplitudes giving a straight line, an ellipse, or 

 a circle. 



On placing one of the small coils at a fixed point on the 

 ring on one side of one of the large coils and the other small 

 coil between the two large coils, I examined the distribution 

 of magnetism between the coils when the same poles or oppo- 

 site poles were opposed to each other. A preliminary experi- 

 ment in which the two small fine coils were on the same side 

 of one of the large coils gave me no indication of nodes, or 

 change of phase. 



When, however, one coil was placed in the manner I have 

 described between the two coils, and one on the opposite side 

 of either of the coils, the following phenomena, which are 

 sufficiently obvious, were observed. With a current of 2500 

 alternations per minute, an ellipse was seen in the phasemeter 

 when the coils were at an equal distance from the ends of 

 either of the large coils. When poles ot the same name were 

 opposed to each other, this ellipse changed from an ellipse 

 with its major axis inclined to the right to an ellipse with 

 this axis inclined to the left or the reverse, thus indicating a 

 difference of phase of 180°. 



At the central point on the iron between the two poles of 

 the same name, the small coil placed there indicated no lines 

 of force threaded through it, and consequently the telephone 

 connected with this coil gave no amplitude, and a line of light 

 either horizontal or vertical was given by the other telephone. 

 When poles produced by the current in the large coils were 

 of opposite sign, there was no appreciable change of phase 

 produced by moving one of the small coils on the iron between 

 these poles ; only the amplitude of the diaphragm of the tele- 

 phone connected with it changed. 



The reason of the change of phase in the first case is 

 obvious. In the case of two poles of the same sign which are 

 opposed, the lines of force are threaded through the coil in 

 one direction on one side of the middle point of the iron 

 between the coils, and in the opposite direction on the other 

 side of this middle point. With poles of opposite sign 

 opposed, the lines of force are threaded in the same direction 

 in whichever way the small coil is moved between these poles. 

 The same phenomena can be observed on straight bars. An 

 interesting manner of showing the change of phase produced 



