122 Mr. Willoughhy Smith [June 6, 



said tliat the induced current is in direct proportion to the speed of 

 the reversals, the battery and spirals remaining the same. It might 

 therefore cause confusion were I not to explain that the deflection 

 would be ten times as great as it was with the lower speed of reversals, 

 but that the scale of the galvanometer not being sufficiently long to 

 record this high deflection, we have what is termed " shunted " a part 

 of the current ; that is to say, between the two ends of the galvano- 

 meter coil we have inserted a length of copper wire, so that the 

 current, on arriving at one terminal of the coil, divides, part going 

 through the coil of the galvanometer, and the rest through the shunt ; 

 the two currents reunite at the other terminal of the galvanometer. 

 By varying the resistance of the shunt, therefore, the desired amount 

 of the current can be sent through the galvanometer. In this case 

 sufficient current passes to keep the beam of light just on the scale at 

 about 86 divisions. We now interpose the sheet of iron as before, 

 and you see the deflection falls as before to about one-half. We with- 

 draw the iron and the deflection returns to its former amount of 86. 

 We now interpose the copper, when the deflection, instead of remain- 

 ing stationary, as in the former experiment, actually falls to 17. 

 We now obtain the following results, viz. : — 



Speed = 1000 reversals per minute. 

 Induced current = 86° deflection. 

 Iron interposed = 40° „ 

 Copper „ = 17° 



Now, the question arose, why does copper, at the low speed of the 

 reversals, apparently have no efi:ect, while at the higher speed it plays 

 so important a part in intercepting the lines of force ? The only 

 solution of the phenomenon which suggested itself to me was that 

 the lines of force have first to polarise the molecules of substances 

 placed in their path before they can pass through them, and in this 

 process time is a very important element to be considered ; for in- 

 stance, at the slow speed there is sufficient time between the reversals 

 for the copper to polarise before the next reversal takes place; 

 whereas at the high speed the copper plate is unable to fully polarise 

 before the next reversal arrives, and then the two induced effects 

 partly blend, and being opposite in direction tend to cancel each 

 other. Now, if this really be the case, the higher the speed the less 

 should be the proportional deflection when experimenting with copper. 

 The time at our disposal this evening will not allow of accurate 

 measurements, or of other substances being experimented upon, but 

 careful and reliable measurements have been made, the results of 

 which are shown on the sheet before you marked 1. It will be seen 

 by reference to these results that the percentage of inductive energy 

 iutercepted does not increase for different speeds of the revcrser in 

 the same rate with different metals, the increase with iron being very 

 slight, whilst with co])pcr the induced current set up is so long in 



