DIFFERENT MODES OF GENERATING CURRENTS. 57 



Experiments on the length of the magnetic cores have not 

 yet been sufficiently multiplied and sufficiently conclusive to 

 enable a very definite law to be formulated with regard to 

 them. Poggendorff, Muller, and several otner physicists have, 

 however, observed in a simple coil with a straight core that 

 the inducing action is stronger at the middle of the core than 

 in any other point, and therefore they have advised that the 

 greatest possible number of spires should be accumulated on 

 this part of the coil, which implies giving to the coils the 

 form of a spindle. This result will be understood if it be 

 considered that the middle of a coil corresponds with the re- 

 sultant of all the dynamical effects of the individual currents 

 of the magnetic core. As to the length of the coils them- 

 selves, it seems that there is an advantage in making them 



FIG. 13. 



rather long to obtain tension, and rather short to obtain 

 quantity. Siemens, however, has contrived a form of induc- 

 tion coil entirely different from the ordinary forms, which has 

 given excellent effects. It is represented in Fig. 13. The 

 magnetic core is made of a cylinder of iron, in which a wide 

 groove is formed longitudinally surrounding the cylinder, and 

 in this the wire of the coil is wound parallel to the axis cf 

 the cylinder, and is bound with bands which prevent it from 

 yielding to the influence of centrifugal force when the bobbin 

 rotates. The parts of the iron cylinder left uncovered form 

 the polar ends of the coil. The bobbin of course revolves on 

 the axis of the cylinder, and in a semi-cylindrical cavity 

 formed in the iron armature fitted to the two poles of the 

 inducing magnet, as shown in Fig. 14. The two ends of the 

 wire of the coil terminate in a commutator shown at the end 



