200 Mr. F. W. Bowden on 



And, remembering that i == \^2 ,i, we get 



I=*Ji*sin*. 



9 



Thus the proper expression for the alteration of the field 

 excitation in ampere turns is 



2In<£ 

 or 



2 \/2/nsin<£. (11) 



To find the total excitation of the field we must add expression 

 (11) to or subtract it from the ampere turns on the field 

 according as the current leads or lags in a generator, and 

 lags or leads in a motor. 



XIII. An Electromagnetic Effect. By F. W. Bowden*. 



SOME time ago, under the direction of Prof. Perry, I was 

 making some experiments suggested by Dr. Larmor. 

 The following curious observations were made, and as 

 Dr. Larmor thinks them distinct from his own object, I beg 

 leave to bring them before the notice of the Physical Society. 

 In fig. 1, N and S are nearly cubical blocks of iron whose 

 edges are about 6 centim. long, forming the poles of an electro- 

 magnet. The field between N and S near the middle was 

 about 1100 C.G-.S. A is the end view of a glass tube, 

 4 millim. internal diameter and about 30 centim. long, con- 

 taining mercury and connecting two large mercury troughs, 

 and a current of about 40 amperes was sent through it. A 

 stand-pipe, B, 2 millim. diameter, indicated the pressure of 

 the mercury. When the current passes in one direction (out 

 from the paper) the mercury rises 15 millim. in B. When the 

 current is reversed, the mercury falls 15 millim. Reversing 

 the magnetic field produces the same effect as reversing the 

 current. 



With the tube in the position shown in fig. 2, the effects 

 were of the same kind but not so marked, the rise and de- 

 pression being about 10 millim. As the tube was lifted more 

 and more out of the field, the effects became less and less. 



The field being vertical as in fig. 3, the mercury rose in 

 the stand-pipe when the current passed, and in whatever 

 direction the current passed ; reversing the current did not 



* Communicated by the Physical Society : read June 28, 1895. 



