THE TELEPHONE. 749 



number of lines, and a positive current in the circuit. This principle was 

 employed by Page in the construction of one of the earliest magneto-electric 

 machines, but it was reserved for Prof. Bell to discover that the vibrations of 

 a tinned iron plate, set in motion by the voice, would produce such currents 

 in the circuit as to set in motion a similar tinned plate at the other end of 

 the line. 



It will help us to appreciate the fertility of that germ of science which 

 Faraday first detected and developed if we recollect that year after year he 

 had employed the powerful batteries and magnets and delicate galvanometers of 

 the Royal Institution to obtain evidence of what he all along hoped to discover 

 the production of a current in one circuit by a current in another, but all 

 without success, till at last he detected the induced current as a transient 

 phenomenon, to be observed only at the instant of making or breaking the 

 primary circuit. 



In less, than half a century, and by the aid of no second Faraday, but 

 in the course of the ordinary growth of scientific principles, this germ, so 

 barely caught by Faraday, has developed on the one hand into the powerful 

 currents which maintain the illumination of the lighthouses on our coasts ; and 

 on the other, into these currents of the telephone which produce an audible 

 effect, though the engine that drives them is itself driven by the tremors of a 

 child's voice. 



Prof. Tait has recently measured the absolute strength of these telephone 

 currents. He produced them by means of a tuning fork vibrating in front of 

 the coil of the transmitter. Before the transmitted note ceased to be audible 

 at the other end of the line he measured by means of a microscope the 

 amplitude of the vibrations of the fork. 



He then placed a very delicate galvanometer in the circuit and found what 

 deflection was produced by a measured motion of the fork. 



Finally he measured the deflection of the galvanometer produced by a small 

 electromotive force of known magnitude. He thus found that the telephone 

 currents produced an audible effect when reversed 500 times a second, though 

 their strength was no greater than what a Grove's cell would send through a 

 million megohms, about a thousand million times less than the currents used 

 in ordinary telegraphic work. 



One great beauty of Prof. Bell's invention is that the instruments at the 

 two ends of the line are precisely alike. When the tin plate of the transmitter 



