DOVE ON THE ELECTRICITY OF INDUCTION. 133 



by its approach to the magnet then destroy each other com- 

 pletely, and there remains only in the wire the effect produced 

 by the magnetism of the iron nucleus. Instead of approaching 

 two spirals coiled in a like direction to both poles of the magnet, 

 two spirals, one right, the other left-handed, can be connected 

 crossways. Upon this principle I had the following apparatus 

 constructed. 



40. The annexed drawing re- 

 presents the rotating keeper of ^ 

 a machine, constructed in other ' 

 respects upon the principle of 

 Saxton's by M. Oertling : it 

 consists of a wooden disc upon 

 a rotating axis, upon which are 

 firmly fixed two hollow coils, 

 the wire of each being 400 feet 



in length, and one-third of a line in thickness. Into these 

 empty wire coils g g and r r, massive iron cylinders d, or bundles 

 of wire d' 13"'-6 in diameter, and 22"'-5 in height can be placed, 

 and by a screw s firmly fastened to the transverse wooden disc 

 of the keeper. To be able to connect both wire coils in the same 

 or alternate direction, the ends of both coils must not be imme- 

 diately connected with the interrupting arrangement of the 

 keeper, but must remain free. A connexion of these ends by 

 wire-clamps is not however convenient, for, if they are not very 

 tightly fastened, and the keeper rotates quickly, they are liable 

 to be forcibly thrown off. I have therefore connected a contri- 

 vance to the keeper, which, as it is intended for compensating, 

 may be called a compensator, and which, by means of two move- 

 able arms x x, admits of both coils being connected in the same 

 or in the alternate direction, and also of only one coil being 

 active. In the first case, the arms rest in the figure upon + +, 



in the second upon , and in the third upon + - . When 



the arms rest in the position + + the connexion is then j!> /3<xban; 



when in the position then it is p cc/3 b an; and lastly, when 



they rest upon + — it is p b a n, in which case it is immaterial 

 whether connexion takes place at the upper or lower plate, 



+ — and 1- are two small copper plates placed one above 



the other, is clamped below the upper one, « below the lower 

 one. IMie point of rotation of the arm x' leads by means of p 



