S6 THE POPULAR SCIENCE MONTHLY. 



in the conductor. It sometimes suits our purpose to augment the 

 transformation of electrical into heat energy at certain points of the 

 circuit when the heat-rays become visible, and we have the incandes- 

 cent electric light. In effecting a complete severance of the con- 

 ductor for a short distance, after the current has been established, a 

 very great local resistance is occasioned, giving rise to the electric 

 arc, the highest development of heat ever attained. Vibration is an- 

 other form of lost energy in mechanism, but who would call it a loss 

 if it proceeded from the violin of a Joachim or a Norman-Neruda ? 



Electricity is the form of energy best suited for transmitting an 

 effect from one place to another ; the electric current passes through 

 certain substances the metals with a velocity limited only by the 

 retarding influence caused by electric charge of the surrounding dielec- 

 tric, but approaching probably under favorable conditions that of radi- 

 ant heat and light, or 300,000 kilometres per second ; it refuses, how- 

 ever, to pass through oxidized substances, glass, gums, or through 

 gases except when in a highly rarefied condition. It is easy, there- 

 fore, to confine the electric current within bounds, and to direct it 

 through narrow channels of extraordinary length. The conducting 

 wire of an Atlantic cable is such a narrow channel : it consists of a 

 copper wire, or strand of wires, five mm. in diameter, by nearly 5,000 

 kilometres in length, confined electrically by a coating of gutta-percha 

 about four mm. in thickness. The electricity from a small galvanic 

 battery passing into this channel prefers the long journey to America 

 in the good conductor, and back through the earth, to the shorter 

 journey across the four mm. in thickness of insulating material. By 

 an improved arrangement the alternating currents employed to work 

 long submarine cables do not actually complete the circuit, but are 

 merged in a condenser at the receiving station after having produced 

 their extremely slight but certain effect upon the receiving instrument, 

 the beautiful siphon recorder of Sir William Thomson. So perfect 

 is the channel and so precise the action of both the transmitting and 

 receiving instruments employed, that two systems of electric signals 

 may be passed simultaneously through the same cable in opposite 

 directions, producing independent records at either end. By the ap- 

 plication of this duplex mode of working to the direct United States 

 cable under the superintendence of Dr. Muirhead, its transmitting 

 power was increased from twenty -five to sixty words a minute, being 

 equivalent to about twelve currents or primary impulses per second. 

 In transmitting these impulse-currents simultaneously from both ends 

 of the line, it must not be imagined, however, that they pass each 

 other in the manner of liquid waves belonging to separate systems ; 

 such a supposition would involve momentum in the electric flow, and 

 although the effect produced is analogous to such an action, it rests 

 upon totally different grounds namely, that of a local circuit at each 

 terminus being called into action automatically whenever two similar 



