7 6 THE POPULAR SCIENCE MONTHLY. 



reaching the earth, and a moiety of it ascending the ground-line at 

 D' counteracts the first weak installment of the other. Then, as each 

 turn of the coil, C, acts the part of the earth on the turn next it, the 

 whole sets up another powerful extra current, which at first forces 

 the full strength of the main current through the recording instru- 

 ment, and ultimately counteracts the accumulated electricity and the 

 second extra current due to the earth. In practice, several such lines 

 are used, and magnets, which are preferable, instead of coils. This 

 occasions a great loss of electricity, but the sensitiveness of the re- 

 ceiving apparatus is such that less than one-fourth of the total strength 

 of the current is sufficient to give a good record. 



The chemical used by Bain in his sensitized paper was ferrocyanide 

 of potassium, which, with the oxide from the iron pen and an extra 

 equivalent of oxygen, forms Prussian blue. The oxygen of the air, it 

 has been found, protracts this action, and thus arises another source 

 of confusion, which is not affected by the device just described. A pref- 

 erable combination, requiring only the protoxide of iron, which is 

 formed immediately by the electricity, is used in the American system. 



One of the most curious of the recent discoveries respecting the 

 chemical action of electricity is that of its usefulness, under certain 

 circumstances, as a lubricator. During Mr. Edison's experiments on 

 the automatic telegraph he perceived that, when using a paper soaked 

 in a certain solution, the pen was apt to slip whenever a discharge 

 occurred. This effect was found to be so marked that a person draw- 

 ing a strip of metal along the paper leaning rather heavily on it 

 finds his hand obliged to move in a succession of jerks when signals 

 are sent by a current powerful enough to overcome the resistance of 

 his body. On this principle, Mr. Edison has constructed a little in- 

 strument in which a style is kept pressed against the paper by springs 

 so as to make a continuous indentation, except when the current is 

 passing. Its record is, therefore, the reverse of that of a Morse regis- 

 ter ; but the " electromotograph," as it is called, differs also from the 

 " Morse " in being the most sensitive recording instrument known. 



Still another of Mr. Edison's inventions is the quadruplex telegraph, 

 the principal aim of which is, not to augment the speed of signaling, 

 but, like the duplex, to allow of several persons using the same wire 

 at one time. In fact, the arrangement may be used as a duplex tele- 

 graph, if required, so that the wire is by it made susceptible of either 

 double or quadruple employ. 



The instruments used are modifications of those of the Morse sys- 

 tem. The " key " has already been shown in Fig. 2, and the changes 

 made to adapt it to the uses of the quadruplex telegraph may be un- 

 derstood from Fig. 5. The essential part of the receiving instrument 

 is an electro-magnet, which is shown in Fig. 2, and consists of a bent 

 bar of soft iron, surrounded at each end by a coil of wire connected 

 with the wire of the line. The current, passing through these coils, 



