358 THE ELECTRIC TELEGRAPH. 



An idea of the gradual perfection to which the covering of 

 submarine wires with gutta-percha has attained, may be 

 gleaned from the relative insulations of the principal cables 

 which have been made. 



The core of the Atlantic cable, made in 1856, had an insu- 

 lation resistance of 12 millions per knot, at a temperature of 

 24Cels. ; the Red Sea and Indian cable, made in 1859, had 

 30 millions per knot ; the Toulon- Algiers cable, made in 

 1860, 60 millions per knot; the Malta-Alexandria cable, 

 made the same year, 100 millions ; the Oran-Carthagena 

 cable, made in 1863, 350 millions ; the Persian Gulf cable 

 the same ; and, lastly, the Atlantic cable, nearly 500 millions 

 per knot. 



The insulation resistance of a cable is measured by one of 

 the following methods : 



(1.) Wheatstone's bridge. 

 (2.) Differential, method. 

 (3.) Deflection. 



77. The method of measuring a cable by means of Wheat- 

 stone's bridge was given when treating of Messrs. Siemens' 

 testing-board. With the customary arrangements, this method 

 is limited to the measurement of resistances of 10 millions. 

 When greater resistances are sought to be measured, it would 

 be necessary, if this method were to be used, to increase the 

 proportion of the two branches of the bridge, or the value of 

 the adjustable side, but which would proportionably reduce 

 the sensibility of the galvanometer. 



78. To avoid this, the system has been introduced of mea- 

 suring the resistance by mean of a differential galvanometer, 

 the magnetic effects of whose coils upon its needle are very 

 different. When this is done, and a single battery used, the 

 intensity in the smaller circuit is obliged to be so great in 

 order to balance the needle, that it endangers warming the 

 coil and increasing its resistance. To remedy this incon- 

 venience, Messrs. Siemens, in their measurements, have 

 sometimes employed separate batteries in the two circuits of 

 the galvanometer. 



Such an arrangement is shown in Fig. 164, in which x is 



