i873-l Magneto-Electric Illumination. 317 



on for eight hours with one of the first machines con- 

 structed, the deviation of the needle of a galvanometer was 

 ahsolutely invariable. Again, a voltaic battery is a compli- 

 cated piece of apparatus ; for each element consists of four 

 separate solid pieces (the outer cell, the porous cell, the 

 positive and the negative element) and two liquids, whilst 

 in most experiments a considerable number of batteries is 

 required. From this multiplicity of parts a voltaic battery 

 is subject to many accidental derangements, which are 

 likely to weaken if not destroy its power. With the 

 magneto-electric machine there is no complication. All 

 the parts are solidly connected together, and no special care 

 is required. 



It must also be remembered that a powerful voltaic 

 battery costs almost as much when it is at rest as when in 

 action. The magneto-electric machine, on the contrary, 

 costs nothing when it is not producing an external current. 

 This may be understood in two senses. It is, of course, 

 evident that when no current is required the rotation of the 

 machine may be stopped ; but it is a remarkable fact that, 

 even when rotation of the armature is still going on, no 

 mechanical force is expended except that necessary to over- 

 come friction, provided the exterior current does not flow. 

 To understand this, let us examine a little more closely into 

 the working of the machine. In the first place, suppose 

 the machine to be in rapid movement, and furnishing a 

 current in an exterior circuit, it will be observed that the 

 armature does not get hot; from this it may be concluded 

 that all' the mechanical force transmitted to the machine is 

 converted into electricity, since none is changed to heat. 

 In the next place, the machine continuing to revolve with 

 the same speed, suppose the exterior circuit to be broken ; 

 still the machine does not rise in temperature, showing that 

 in this case there is neither production of heat nor elec- 

 tricity, and consequently no waste of mechanical force. 

 From the way in which the currents in the armature are 

 generated, when there is no exterior circuit along which 

 they can flow, they neutralise one another, and keep in such 

 perfect equilibrium that there is absolutely no circulation, 

 and consequently no heating. 



If the Gramme machine is set in motion by a force just 

 sufficient to turn it *with a definite velocity when the 

 exterior current is flowing, and if the outer circuit is 

 suddenly broken, the machine is seen to acquire an in- 

 creasing velocity, showing that the mechanical force applied 

 to it, being no longer capable of going off as electricity, 



vol. in. (n.s.) 2 T 



