HISTORY, &c., OF INDUCTION INSTRUMENTS. 



299 



is a matter of great interest to be able to compare the effects pro- 

 duced under circumstances that are very different, and moreover 

 well defined. But it 

 was, perhaps, not wise 

 to seek the means of 

 this comparative study 

 in a complex apparatus, 

 in which the building 

 up of different parts 

 above one another 

 micjht create a diver- 

 sity of conditions diffi- 

 cult to estimate, — such 

 as differences of dis- 

 tance, of intensity, &c. 

 In the performance of 

 chamber experiments 

 it is much more simple, 

 more exact, and also 

 more economical, to 

 have a number of reels 

 in which a single con- 

 dition varies from one 

 to another, and all sup- 

 plied by a single inter- 

 rupter in which the 

 motor is independent 

 of the active current, 

 as arranged in some 

 models of the inter- 

 rupter of Foucalt." 



(d). The apparatus 

 ofDu Bois Reymond, or 

 of Siemens and Halske. 

 — " The battery cur- 

 rent is brought to the 



^ A, A', biuding-screws, to receive the 

 electrodes of one of the Ijatteries ; a, a, 

 binding-screws to receive the electrodes 

 of the other battery. B, reel on which is 

 coiled the fine wire ; b, small reel of fine 

 wire containing within it an electro- 

 magnet, whicli regulates the circuit of 

 the preceding. B', reel on which is coiled 

 the thick wire ; b', a small reel of thick 



wire, containing an electro-magnet, which 

 regulates the interruptions of the circuit 

 of the reel B', when this is traversed by 

 the battery-current. E, head of the 

 stem whicli contin^^es the central electro- 

 magnet, and forms a handle for it. C, 

 commutator. D, handle, placing the ap- 

 paratus in communication with one or 

 other of the batteries. 



