134 DOVE ON THE ELECTRICITY OF INDUCTION. 



to the iron roller upon which the interrupting springs slide* ; the 

 point of rotation of the arm x leads by means of the whole coil 

 of wire b a through n to the other. 



When the compensator has the position + +, and the spirals 

 are empty, a current is obtained induced by the approach of a 

 closed conductor to a steel magnet. In the position of the com- 

 pensator w^th empty spirals, an equilibrium of current is 



established for physical, chemical and physiological tests. If, 

 however, one of the coils then contains a solid iron cylinder, 

 a current is obtained, induced by the sole action of the magnetic 

 polarity produced in the iron cylinder. The axis of the keeper 

 must of course turn perfectly true upon a conical point, in con- 

 cave conical hollows, because the masses now set in motion by the 

 rotation are no longer symmetrically placed with relation to the 

 axis of rotation. If, on the contrary, the compensator having 

 the position -f + , an iron cylinder is placed in each of the coils, 

 then a current is obtained induced by the assumption of magne- 

 tic polarity by these cylinders, and by the approach of a closed 

 copper wire to a steel magnet, producing therefore the most 

 powerful action. As this arrangement is the same as that usu- 

 ally adopted in the construction of Saxton's machine, I have 

 made use in these experiments of that instrument, as it is de- 

 picted (at fig. 7) and described in the sequel, § 70, in which the 

 cross beam is also of iron, and the wire can be so connected that 

 both coils are joined at their two ends in a kind of parallel con- 

 nexion. 



The current excited in this triple manner in the wire coils of 

 the keeper was now circulated in the inner spirals of the third 

 differential inductor, of which the inner and outer spirals were 

 composed of wire 400' in length. This apparatus was so sensi- 

 tive when both inductions were combined, that a tube of thin 

 sheet nickel produced a distinctly positive action, and the nega- 

 tively distui'bed equilibrium of the differential inductor could be 

 traced by means of a bar of brass placed in one of the spirals, 

 connexion being established by the mouth. The experiments 

 gave the following results, 



41. An unenclosed bundle of wire and one placed in a cut 

 tube very nearly compensated each other physiologically. If, 



* The more accurate description of tliese rollers, depicted in Plate J. fig. 7, 

 V, wj, will be given afterwards, ^ 7^ • 



