ELECTRIC LIGHT. 265 



In this way it can be used to find the work done by a fluid pressure recipro- 

 cating engine, or the energy transmitted by a shaft or belt from one part of 

 a factory to another. By making the wheel very small and light, the strength 

 of an electric current may be continuously measured, if the disc is inclined 

 by means of the needle of a galvanometer in circuit. Mr. Boys has con- 

 structed on the same principle an electric energy meter, which integrates the 

 product of the strength of current and the difference of potential between 

 two points with respect to time. In it the current is passed through a pair 

 of concentric solenoids or coils of wire, and in the annular space between 

 these is hung a third solenoid, the upper half of which is wound in the 

 opposite direction to its lower half. By the use of what Mr. Boys calls 

 " induction traps " of soft iron, the magnetic force is confined to a small 

 portion of the suspended solenoid, and by this means the attracting force of 

 the fixed solenoids upon it is independent of position. The middle solenoid 

 is hung from the end of a balance beam, and its motion is retarded by a 

 counterweight, which admits of regulating the meter to give standard measure 

 as a clock gives standard time. The motion of the beam is caused to incline 

 the integrating wheel, and the rotation of the cylinder gives the energy 

 expended in foot-pounds by means of an indicator or diagram, as the case 

 may be. The object in giving an equal number of turns in opposite direc- 

 tions to the suspended solenoid is to render the instrument insensible to 

 external magnetic forces. 



We have, in a former portion of this work, explained the construction 

 of the telephone and phonograph with other inventions to make sounds 

 audible at a distance, so we need not repeat the explanations here. A brief 

 reference to them will, however, be found in this chapter, in which the 

 electro-magnet and the methods of lighting by magneto-electric machines 

 are treated of. We will proceed to give some particulars concerning the 

 electric light before considering the means by which it is produced, as such 

 an arrangement is more convenient. 



The light is very easily produced by uniting and then separating the 

 terminals of a strong battery. The passage of the electric current induces 

 intense heat and a most brilliant light. But if this were continued the wires 

 would melt, and therefore some non-fusible substance is placed at the ends 

 of the wires, which will be at once a conductor and infusible. Now in gas- 

 carbon (the deposited substance found in gas retorts) we have a substance 

 suited to these conditions. The carbon is heated to an intense brightness, 

 and particles of it are passed across the arc of flame almost in a state of 

 fusion. Combustion does not actually take place, because it has been proved 

 that the wires will give out light under water, and in the vacuum of an air-pump 

 the light is even increased, so that had the oxygen of the air any part in the 

 production of the light it would not remain unaffected under these conditions. 

 The heat arising from this Voltaic arc is intense, and even platinum may be 

 fused with the assistance of the gas carbon. The carbon points are of course 

 liable to be worn away, and one side more than another. The positive pole 



