1891.] on Electro-magnetic Bepulsion. 297 



in illustrating the now familiar interactions of two circuits traversed 

 by currents in the same or opposite directions. Holding a circular 

 coil traversed by a continuous electric current near to a similar cir- 

 cuit free to move, we are well aware that when the circuits are 

 parallel to each other there is an attractive force between them if 

 the currents in adjacent parts of the circuits flow in the same 

 direction, and a repulsive effect if they flow in the opposite. This 

 is the electro-dynamic action discovered by Ampere and utilised in 

 the construction of instruments for the measurement of electric 

 currents in practical w^ork. If one such conducting coil, such as that 

 of the electro-magnet now before me, is traversed by an alternating 

 current, and the other is simply a closed circuit or coil placed a little 

 distance off, but in its field, I feel sure I am free to assume that all 

 here are aware of the effect which will be produced. The closed 

 circuit becomes the seat of an alternating induced current, which, 

 if our inducing current is sufficiently powerful, can be made to render 

 itself evident by illuminating a small incandescent lamp placed in 

 the secondary circuit.* Notice, however, that in performing the 

 experiment the secondary circuit must be so placed that the magnetic 

 lines of force of the primary coil perforate through the secondary 

 circuit. If the secondary circuit is held in such a position that the 

 reversal of direction of the primary current causes no reversal of 

 direction of the magnetic field traversing the secondary circuit, 

 because it is not linked with any of the lines of force of the primary, 

 the secondary circuit is no longer the seat of any induced current. 



Note also that this electro-magnetic induction, whilst taking 

 place across space, is not interfered with by the interposition of a 

 non-conducting screen. The magnetic induction passes freely 

 through an inch board or a plate of glass, but if we interpose a 

 thick copper sheet (Fig. 1), we cut off the secondary current or screen 

 the secondary circuit from the inductive action of the primary. The 

 secondary coil is, in fact, an electro-magnetic eye which can see through 

 an inch of wood, but to which a sheet of zinc is semi-transparent, 

 and a thick sheet of copper quite opaque, as far as the electro- 

 magnetic radiation of the wave-length employed is concerned. The 

 rapid heating of this copper screen makes us aware that the secondary 

 currents are now being induced in the copper sheet in the form of 



* The experiments here referred to were mostly performed with an alter- 

 nating-current magnet, having a core of divided iron about 3 inches in diameter 

 and 12 inches long, excited hj a Siemens alternating-current dynamo, kindly 

 lent for the occasion by Messrs. Siemens Bros., giving a current at an electro- 

 motive force of rather less than 200 volts. A small shelf around the core a little 

 above the middle serves as a support for rings, &c., to be projected. The 

 performance of these experiments on a scale suitable for large audiences requires 

 trom 10 to 15 horse-power at least, and can hardly be shown well unless the 

 alternator can provide a current of 100 amperes at 100 volts, available at the 

 moment of maximum demand. The magnet of course takes very little actual 

 current until the metal plate or ring is held near it, when the impedance is 

 immediately reduced. 



