326 PROCEEDINGS OF THE AMERICAN ACADEMY 



way, while the magnetic wave is rising within it, and the other way, 

 while falling ; giving no effect, as a whole, upon the galvanometer. 

 The second (adjustable) commutator, already alluded to, made it 

 possible to "redress"* these currents, so that they should all pass 

 through the galvanometer in either direction. In fact, it was almost 

 impossible so to adjust it that the current should not be turned, on 

 the whole, a little one way or the other. 



To avoid jarring, and direct action of the magnetism upon the 

 galvanometer, whose deflections were to be noted, the machinery was 

 removed as far as possible from the observer ; while an assistant, 

 according to direction, varied the angle between the two commuta- 

 tors, by means of the sliding-piece already alluded to, until the aver- 

 age deflection of the galvanometer was reduced to zero. Now the 

 effect of the commutator in the secondary circuit is to add together 

 all the pulses, positive and negative, which pass through it, in 

 either of its two positions, to the galvanometer ; and the symmetry 

 of the wave, as regards positive and negative displacements, shows that 

 the pulses are all alike ; hence, each must be equal to zero when the 

 total effect is zero. Since the magnitude of the pulse for a given 

 interval of time is proportional to the rise or fall of the wave during 

 that time, this rise or fall must also be equal to zero ; that is, the 

 initial and final dis23lacements are identical : and hence (again from 

 principles of symmetry) it is evident that, if there be no deflection 

 of the galvanometer, every magnetic wave must have reached its meau 

 height within the secondary coil at the instant of action of the (ad- 

 justable) commutator included in the circuit; so that comparison of 

 these angles of adjustment for different velocities of the machinery, 

 different distances between the two coils, and for different bars served 

 to determine the various rates of propagation of the magnetic wave 

 under these different conditions. 



Earlier experiments gave discordant results, owing partly to imper- 

 fect contact within the commutator (which was afterward remedied 

 as far as possible), but especially on account of the change of " mag- 

 netic conductivity " (if I may use the term) which the bar appeared 

 to undergo after a few minutes of rapid and powerful magnetization 

 and demagnetization. In one case, the conductivity appeared in con- 

 sequence to increase two or three fold. 



I should have been at some pains still further to investigate (or at 

 least to confirm) this sensitiveness of soft iron to the influence of a 



* In the sense of tlie French word, redresser. 



