398 THE ADDRESSES, LECTURES, ETC., OF 



which he showed would be multiplied indefinitely. The machine 

 which I placed before the Koyal Society in the year 1865 is now 

 before you ; it has done a great deal of useful work since, having 

 been employed at the Telegraph Works at Woolwich to magnetise 

 steel bars to make them permanent magnets ; we will set it at 

 work. The machine is now being worked by a current, for the 

 dynamo-machine can serve either as a power-giving machine, or as 

 a power-receiving machine. If you pass a current through these 

 coils, you transform electric into mechanical energy ; if, on the 

 other hand, you turn the armature forcibly round at the same 

 speed as before, you produce nearly the same current which was 

 originally taken in driving it. 



At the time the dynamo principle was first announced, great 

 interest was expressed in its behalf by my friend, the late Professor 

 Clerk Maxwell, who saw in the mutual convertibility, by the 

 same piece of mechanism, of mechanical into electrical effect, and 

 vice versa, a great practical proof of the correlation of physical 

 forces. The phrase, attributed to him in popular essays, viz., 

 that one of the greatest discoveries of the present century was the 

 reversibility of the Gramme machine, must however be received 

 with great reserve, considering that the particular machine with 

 which the name of Gramme is associated was not brought out 

 until five years after the dynamo-electric principle of action had 

 been established. 



It is a remarkable feature connected with dynamo-electricity 

 that the second law of thermo-dynamics is not involved. Theo- 

 retically speaking, a certain amount of mechanical force can be 

 converted entirely into electrical force ; and electrical force 

 theoretically speaking can be so converted into dynamical force. 

 Practically speaking, of course, that is not so. There are neces- 

 sarily losses, and one of these, which is self-evident, is that the 

 current passing through the coils must produce resistance, and 

 electrical resistance, wherever it appears, converts electric energy 

 into heat energy. Again, the iron bars which are magnetised 

 and demagnetised at every half-revolution set up currents in 

 themselves, for it is natural that instead of the current flowing 

 only through the convolutions of the wire, the iron itself being a 

 metal, some current will be set up in it, and this current so set up 

 produces the effect of heating the iron ; it simply circulates round 



