164 ANNUAL OF SCIENTIFIC DISCOVERY. 



As kte as December, 18,53, despatches were written direct through from 

 New Orleans to Philadelphia and New York, the weather being cold and 

 the earth frozen. In so doing, the only connector or repeater used was an 

 insulated screw on the back of the register, invented by Mr. McRea, of 

 Philadelphia. But this distance would require at least 30 Grove's cups, 

 of a pint each, for every 100 miles; making about 480 cups, or 240 each 

 side. If a copper and zinc battery were employed, the number would 

 have to be increased to about 30 or 40 cups, every 100 miles ; but, even 

 with this large battery, the expenses would be less than with Grove's bat- 

 tery. In preparing the batteries, it is even possible to determine mathe- 

 matically beforehand the amount of resistance and the force necessary to 

 overcome it, and thus to proportion the number and size of the plates to 

 the distance to which the wires extend. Large wires are better conductors 

 than small ones ; copper is a much better conductor than iron ; and as a 

 thinner wire answers the purpose of conduction, it may be much more 

 easily insulated. The several conditions may all be calculated from the 

 beautiful formula of Ohm. 



In some recent experiments of Prof. Farraday, that distinguished philoso- 

 pher, by some of the experiments he obtained, has thrown much liglit 

 upon the action of voltaic electricity in the submerged wire of the electric 

 telegraph. 



He first determines by actual experiment, that when copper wire is per- 

 fectly covered with gutta-percha, so high is the insulation, that in 100 

 miles of such wire, when fully charged by an intensity battery of 350 pairs 

 of plates and submerged in water, the deflection of a delicate galvanome- 

 ter was not more than 5. The great perfection in the covering of the 

 wire may be judged of by this fact alone. The 100 miles of wire were one- 

 sixteenth of an inch in diameter ; the covered wire was four-sixteenths ; 

 the gutta-percha on the metal was considered as 0.1 of an inch in thick- 

 ness. There could not be any better proof than this, that gutta-percha is 

 one of the best ins ulating agents we have. 



He experimented with the subterraneous wires which exist between 

 London and Manchester ; and when they were all connected together so as 

 to make one series, they made almost the distance as determined by Lieu- 

 tenants Eerryman and Maury between the Irish coast and Newfoundland, 

 being 1,500 miles ; and having introduced galvanometers at intervals of 

 about 400 miles, he found that, when the whole 1 ,500 miles were included, it 

 required two seconds for the electric stream to reach the last instrument 

 which was placed at the end. In this instance the insulation was not as 

 perfect ; still the result shows that it will require a little over two seconds 

 to cross the Atlantic by telegraph, which is about the rate of 750 miles in a 

 second, which result is far below those obtained by the London and Brus- 

 sels telegraph, which is stated at only 2,700 miles in a second, even with a 

 copper wire ; while it will be remembered that Wheatstone, in 1834, with 

 copper wire, made the velocity of the electric current 288,000 miles per 

 second a considerable difference. 



