1884.] on Volta-Electric and Magneto-Electric Induction. 121 



from a battery ; B is so connected to the galvanometer and a reverser 

 as to show the deflections caused by the induced currents, which are 

 momentary in duration, and, in the galvanometer circuit, all on the 

 same side of zero ; for, as the battery current, on making contact, 

 produces an induced current in the reverse direction to itself, but in 

 the same direction when broken, of course the one would neutralise 

 the other, and the galvanometer remain unaffected. To obviate this, 

 the galvanometer connections are reversed with each reversal of the 

 battery current, and thus a steady deflection is produced. 



Perhaps, for the information of those not acquainted with the con- 

 struction of a galvanometer, I ought to explain it more fully. The 

 one I am about to use consists of a coil of very fine silk-covered 

 wire, in the centre of which is suspended a very small magnet ; the 

 ends of the coil of wire and the ends of spiral B are connected re- 

 spectively together, thus forming a metallic circuit, one part of which 

 is wound into a coil and the other into this spiral. Call to mind 

 Oersted's experiment, and it will be readily understood that when a 

 current of electricity is flowing in this metallic circuit, the magnet 

 will be influenced in magnitude according to the amount of current 

 thus flowing. The movements of the magnet, however, would be too 

 small to be seen unless watched very closely ; therefore fixed to it is 

 a very small concave mirror, on to which a beam of light from the 

 lamp is thrown, and the mirror reflecting this on to the scale, will, I 

 hope, enable a]l to see that somewhat broad beam move in accord- 

 ance with tlie movements of the magnet. Reversed currents at the 

 speed of 100 per minute will now be passed through spiral A, and 

 you will observe that the induced currents in B give about 28 

 divisions on the scale of the galvanometer : we will note this on the 

 black-board. Now, we place this plate of iron midway between the 

 two spirals, and you observe the deflection on the scale is reduced to 

 about one-half, or in round numbers to 15, showing clearly that the 

 presence of the iron plate has in some way influenced the previous 

 effects. We now remove the iron, when you see the deflection re- 

 turns to its original amount of 28 divisions ; and if I now interpose 

 a similar sheet of copjDer, the interposition does not alter the deflec- 

 tion. The results of this experiment are therefore as follows : — 



Speed = 100 reversals per minute. 

 Induced current = 28° deflection. 

 Iron interposed = 15° „ 

 Copper „ = 28° 



I may here state that, up to this point, the results of my experiments 

 confirm those of Faraday, viz. that all dielectrics and diamagnetic 

 metals appear in no way to interrupt or interfere with the lines of 

 force. 



Now, let us repeat this experiment with the speed of the reversals 

 increased ten times, or to 1000 per minute ; the spirals are in the same 

 position as before, and the deflection is now about 86. I have already 



