450 



NATURE 



[April?,, 1875 



THE PROGRESS OF THE TELEGRAPH * 

 II. 



ELECTRIC force pervades all matter. Our planet and 

 the atmosphere surrounding it are vast storehouses 

 of electrical energy in a constant state of unstable equi- 

 librium. Electricity is one of the forces of nature, and 

 may be developed in various ways and under various 

 conditions. The aurora, the thunderstorm, and the 

 earth's magnetism, are each grand displays of electrical 

 force upon a vast scale. Electrical energy may be ex- 

 cited by chemical action, friction, heat, induction, mag- 

 netism ; and currents of electricity so obtained may be 

 employed for telegraphic purposes. Thermo-electricity, 

 as the name implies, is that generated by electric currents 

 in metallic bodies by the disturbance of the equilibrium 

 of temperature, the essential conditions being, that the 

 extremities of the dissimilar metals should be in opposite 

 states as regards temcerature. The discovery of thermo- 

 electric currents is due to Seebeck of Berlin in 1821 ; 

 the generation of electric currents by the application of 

 heat to a pile or series of dissimilar metals, however, 

 remained in abeyance until the researches of Nobili and 



Melloni, who constiucted the thermo-electric pile, con- 

 sisting of alternate parallel bars of bismuth and antimony, 

 placed side by side. Fig. 10 is a representation of the 

 thermo-electric pile as arranged by Melloni. The brass 

 frame on the left contains the compound bars, the wires 

 from the antimony and bismuth poles being connected 

 to a galvanometer, shown on the right-hand side ; the 

 quantity of electricity passing from the poles of the pile 

 (regulated according to the difference of temperature of 

 the bars) causes the needle of the galvanometer to be 

 deflected. With thermo-electric currents the quantity of 

 electricity developed depends upon the difference of the 

 temperature of the two poles of the dissimilar metals ; the 

 currents may be so delicate that a difference of temperature 

 equivalent to TjTriith part of a degree may be measured. 



Frictional electricity, as the name implies, is that pro- 

 duced by the rubbing together of certain substances. An 

 ordinary form of the frictional electrical machine is shown 

 at Fig. II. It consists, first, of a hollow glass cylinder 

 supported on brass bearings resting upon glass rods ; and 

 then of an exciting rubber of a cushion of leather stuffed 

 with horsehair ; this is mounted on glass supports, and the 

 amount of pressure on the cylinder is regulated by screws. 



- Ihermo-clectnc pile, producing electric current by difference ol teiiiperau 



A flap of oiled silk is attached to the rubber to prevent 

 the dissipation of the electricity from the surface of the 

 cylinder before it reaches the points of the prime con- 

 ductor, which draw the electricity from the glass cylinder 

 on the oiher side. On turning the cyhnder the friction of 

 the cushion occasions the evolution of electricity, the 

 produciion of which is more rapid when the surface of 

 the rubber is smeared with a metal amalgam. When the 

 cylinder machine is arranged for the development of 

 either positive or negative electricity the conductor is 

 placed with its length parallel to the cylinder, and the 

 points project from its side as shown in the figure. The 

 negative conductor supports the rubber and receives from 

 it negative electricity by coiiiDiitniiatiou, and not by in- 

 duction, as is the case with the positive conductor. If it 

 is desired to accumulate positive electricity, a chain must 

 be carried from the negative conductor to the ground ; if, 

 on the other hand, negative electricity is required, the 

 conductor must be placed in communication with the 

 e trth, and the rubber insulated. 



* Continued from p. 392. 



For the purpose of telegraphic transmissions, the cur- 

 rent obtained from chemical action, or from a permanent 

 magnet, is generally employed, and will be sufficient for 

 the purposes contemplated in the present summary. The 

 laws and phenomena that come into play during the pro- 

 pagation of an electric current require examination. 



Electricity may be thus developed in the form of either a 

 quantity or an intensity current, according to the arrange- 

 ment of the elements composing the battery. A quan- 

 tity current is one which, as its name implies, has great 

 surface development. An intensity current is one of 

 series development and of high tension. Quantity and 

 intensity in an electric current may be combined together 

 in different proportions, according :o the work required 

 to be performed. 



As an example, suppose a battery or pile of twelve ele- 

 ments (Fig. 12), each element consisting of a carbon and 

 zinc plate immersed in a glass jar containing for the exciting 

 fluid a saturated solution of common salt. Now, if the twelve 

 carbon plates of the series are all connected together by a 

 common wire, and the twelve zinc plates are similarly 



