396 THE ADDRESSES, LECTURES, ETC., OF 



iron head of the armature is separated from the line of poles of the 

 permanent magnets, i.e., each time it makes a half -revolution, there 

 is a severance due to each of these magnets. Therefore if there 

 are eight bars, we have on the one side of the electro-magnet the 

 joint effect of eight severances, and on the other side a similar 

 amount of effect. So that for each half-revolution we get the 

 result of sixteen such sparks as that shown in Faraday's experiment, 

 and if this can be repeated at a very rapid rate we may get sixteen 

 sparks perhaps ten times in a second. We will now connect the 

 current, not with the dynamometer, because it would not be so suit- 

 able, but with one of these instruments which are generally used 

 for exploding mines, and I will, with your permission, explode a 

 mine. Instead of one we might have ten or twelve mine-exploders 

 in a series. You observe a very powerful instantaneous current 

 resulting from the action of the machine. 



A further step in the development of magneto-machines was 

 furnished by Mr. Wilde of Manchester, by substituting for the 

 steel magnets electro-magnets excited by the current from a sepa- 

 rate magneto-machine furnished with the Siemens armature. By 

 this arrangement Mr. Wilde was able to realise much more powerful 

 effects than could have been obtained previously. 



Another form in which the Faraday or induced current manifests 

 itself is in an induction coil, and this also represents an essential 

 action which we should realise before going any further. In an 

 induction coil one spiral of insulated wire is put within another, 

 both upon an iron centre. When a bar of iron is surrounded, as 

 in this instance, with wire, through which a current is passed, the 

 bar becomes a magnet ; and if outside the wire of the primary 

 coil, as it is called, fine wire is wound, a current is induced in the 

 secondary coil which is of high potential, or tension, according to 

 the number of turns which the wire makes round and round the 

 bar ; therefore if a current of very high potential is wanted, very 

 thin wire has to be taken and coiled round a great number of 

 times, whereas if a current of larger quantity and small potential 

 is required, a thicker wire has to be used, having only a small 

 number of turns. If I were to take a thick wire and make 

 the same number of turns, the outer convolutions would be too far 

 away from the magnet to produce energetic action ; I am there- 

 fore limited by the space at my disposal round this bar of iron in 



