CHAP, v.] ELECTRICITY AND MAGNETISM. 



289 



the coils into two parts wound on, as in Figs. 1 1 7 and 

 1 1 8, near the poles. A special form of electromagnet 



Fig. 118. 



devised by Ruhmhorff for experiments on diamagnetism 

 is shown in Fig, 127. The great usefulness of the 

 electromagnet in its application to electric bells and 

 telegraphic instruments lies in the fact that its mag- 

 netism is under the control of the current ; when circuit 

 is " made " it becomes a magnet, when circuit is 

 " broken " it ceases to act as a magnet. 



33O. Lifting -power of Electromagnets. The 

 lifting-power of ah electromagnet depends not only on its 

 " magnetic strength," but also upon its form, and on the 

 u 



