THE ELECTRIC TELEGRAPH. 



and collected in any desired quantity, and with any required 

 intensity. Among these, that which has proved to be the 

 most efficient for telegraphic purposes is the GALVANIC or VOLTAIC 



BATTEEY. 



17. This apparatus is to the electric telegraph what the boiler 

 is to the steam-engine. It is the generator of the fluid by which 

 the action of the telegraphic machine is produced and maintained. 

 It supplies the fluid in any required quantity and of any desired 

 intensity. As the boiler is supplied with expedients by which 

 within practical limits the quantity and pressure of the steam 

 may be varied, according to the exigences of the work to which 

 the engine is applied, so the voltaic battery is provided with 

 expedients by which the quantity and .intensity of the electric 

 fluid it evolves can be varied according to the distance to which 

 the intelligence is to be transmitted ; and the form, whether 

 visible, oral, written, or printed, in which it is required to be 

 delivered at the place of its destination. 



18. The electric fluid being thus produced in sufficient quantity, 

 it is necessary to provide adequate means of transmitting it to a 

 distance without exposing it to any cause of injurious dissipation 

 or waste. 



If tubes or pipes could be constructed with sufficient facility 

 and cheapness, through which the subtle fluid could flow, and 

 which would be capable of confining it during its transit, this 

 object would be attained. As the galvanic battery is analogous 

 to the boiler, such tubes would be analogous in their form and 

 functions to the steam-pipe of a steam-engine. 



19. The construction of such means of transmission has been 

 accomplished by means of the well-known property of the 

 electric fluid, in virtue of which it is capable of passing freely 

 over a certain class of bodies called CONDTJCTOES, while its move- 

 ment is arrested by another class of bodies called NOX-COKDUCTOES 



Or INST7LATOES. 



The most conspicuous examples of the former class are the 

 metals; the most remarkable of the latter being resins, wax, 

 glass, porcelain, silk, cotton, dry air, &c. 



20. Now if a rod or wire of metal be coated with wax, or 

 wrapped with silk, the electric fluid will pass freely along the 

 metal, in virtue of its character of a conductor ; and its escape 

 from the metal laterally will be prevented by the coating, in 

 virtue of its character of an insulator. 



The insulator in such cases is, so far as relates to the electricity, 



a real tube, inasmuch as the electric fluid passes through the 



metal included by the coating, in exactly the same manner as 



water or gas passes through the pipes which conduct it ; with this 



120 



