MAGNETO-ELECTRIC ILLUMINATION. 593 



Messrs. Chance, and through which the rays are sent in parallel lines. 

 It is 21 inches in diameter, and is composed of lenses, surrounded by- 

 annular prisms, the centre part refracting the rays and the outer rings 

 reflecting them. Should the electric light be adopted, a special lens 

 will be constructed, by means of which the rays will be diffused 

 through an arc of 180, instead of being sent in one direction only. 

 The cost of this electric light is at present estimated at 10c?. per hour. 



It may be of interest if we consider some matters of scientific in- 

 terest in connection with this machine. In the first place, it possesses 

 an enormous advantage over the voltaic battery in the absolute con- 

 stancy of the current so long as the velocity of rotation is uniform. 

 In an experiment carried on for eight hours with one of the first ma- 

 chines constructed, the deviation of the needle of a galvanometer was 

 absolutely invariable. Again, a voltaic battery is a complicated 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, while in most experiments a considerable number of 

 batteries is required. From this multiplicity of parts a voltaic bat- 

 tery 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 pro- 

 ducing 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 overcome friction, provided the ex- 

 terior 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 cur- 

 rent 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 electricity, and con- 

 sequently no waste of mechanical force. From the way in which the 

 currents in the armature are generated, when there is no exterior cir- 

 cuit along which they can flow, they neutralize 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 

 vot.. ni. 38 



