Mr. Watkins on Magneto-electric Induction. 109 



o 



periment. We invite him to observe if the results he obtains 

 in estimating the slowness of the transmission of the magnetic 

 action accord with those which Messrs. Herschel and Bab- 

 bage (in following another method) have found to be the case 

 in England." 



It will be observed that in M. Plateau's experiment a 

 given velocity of rotation in the one magnet gives equal velo- 

 city of rotation to the second magnet. Increase the velocity, 

 and the magnet acted upon, instead of revolving, vibrates, and 

 the amplitude of the arc of vibration is greater or less as the 

 velocity of the rotating magnet placed under it is increased or 

 decreased. Again, we have another velocity more rapid, which 

 with its speed does not affect the suspended needle at all, and 

 it remains in its natural position just the same as if no moving 

 magnetic body were in its vicinity. 



The revolving magnetic needles to which I have called 

 your attention, when they are all fairly started with the same 

 velocity of rotation (or nearly so), act well, and continue to 

 rotate merrily. But if the main revolving magnet (I mean 

 the one which is affixed to the axis carrying the points, 

 which by dipping into the mercury cup, hereafter to be de- 

 scribed, cause the change in the poles of the electro-magnet 

 more or less,) is in very rapid motion, then those magnetic 

 needles which do not acquire the required speed at the mo- 

 ment of starting simply oscillate. The amplitude of the arcs 

 of oscillation, depend I conceive, as indeed is made apparent, 

 on the same cause with that which effected the phaenomena 

 recorded by M. Plateau. I have seen the magnetic needles 

 frequently in perfect repose, at other times slightly tremulous, 

 when the main revolving permanent magnet was rotating ra- 

 pidly, affecting of course the rapidity of the change (or per- 

 haps partial change) in the polarity of the electro-magnet. 



The main revolving permanent magnet does not influence 

 the revolution of the magnetic needles, as might be supposed 

 by some persons on first inspection of the apparatus, for it 

 may be put in rotation in an opposite direction: we may 

 therefore justly conclude that the inductive influence of the 

 electric current on the soft iron, is the principal cause of the 

 continuous rotation of one and all of the rotating magnetic 

 needles. Moreover, this apparatus, with M. Plateau's experi- 

 ment, tends to prove that Messrs. Herschel and Babbage's 

 law is just, viz. that time is an essential element of induction. 



With these views it may with propriety be asked how it is 

 that some or all of the magnetic needles, having acquired the 

 same velocity with that of the main revolving permanent 



