1839.] Telegraphic Signals by induced electricity. 719 



In Davy's telegraph the needles carry slight screens which conceal 

 illuminated letters or numbers — on deflecting the needle the signal is 

 disclosed. 



Soon after the discovery of the deflection of the needle, several 

 attempts were made to establish by its use, the laws of action of the 

 battery. Ritchie attempted to prove that the deflection was in the 

 direct ratio to the surface of zinc acted on in the battery. Thus 

 supposing the conductors unchanged, and that by the corrosion of one 

 superficial inch of zinc a deflection, say of 5° be obtainable, the corro- 

 sion of two superficial inches will give a deflection of 10°. Were this 

 assertion supported, a single galvanometer would give us all the signals 

 we could require. It is now however proved that the supposed law 

 by no means holds good. It is quite true that we may double or 

 treble a given deflection, or that we may by direct experiment propor- 

 tion the voltaic force to the deflection required, but such experiments 

 are only fit for performance in the closet or laboratory, — require such 

 careful adjustment and observation — and are, moreover, so exceedingly 

 delicate, and take so much time in recording, that they become quite 

 unsuitable for the rapid transmission of telegraphic signals. 



In the preceding arrangements in which several galvanometers 

 were used, we have manifestly all that we require within the distances 

 to which experiment has yet reached. But the expense of wire next 

 presents itself as a motive for endeavouring to improve the system by 

 diminishing the number of the wires. To render this intelligible, of 

 the copper bell wire best suited for these experiments, each mile costs 

 276 rupees. 



Steinheils of Munich, the most recent writer on this subject, pro- 

 poses either of two very ingenious methods. The first is causing the 

 galvanometrical needle to terminate in a fountain pen, the tip of which 

 touches and marks a strip of paper revolving by clockwork ; — according 

 to the number of dots a letter or numerical signal can be obtained. The 

 second plan is the employment of the tip of the needle to strike a bell, 

 when the number of strokes in a given time afford the requisite signal. 



The galvanometer moreover has been rendered so exceedingly 

 delicate in its indications, that very feeble electrical forces will succeed 

 in producing deflections. The electricity evolved by holding up the 

 hand before a disk composed of bismuth and antimony, caused in an 

 instrument contrived by Dr. Page, of Baltimore, a deviation of fifty 

 degrees. In a galvanometer in my possession, constructed by Messrs. 

 Watkins and Hill, the action of a drop of acidulated water on a zinc 

 wire the size of a pin, and opposed to a copper element of equal size, 

 urges the needle through a quarter of a circle. Moreover the differen- 



