.S7/? U'H.IJAM SIEMENS, F.R.S. IJ 



detect exceedingly small traces of electric current ; but since then 

 I 'roft'ssor Thomson has brought out a reflecting galvanometer of 

 still greater delicacy, which is now used in preference. In testing 

 the Marsala cable we have introduced another instrument, which 

 combines the functions of the galvanometer and of the bridge.. It 

 is a differential galvanometer, one coil of which is mounted upon 

 a carriage, and is moved by a micrometer screw to such a point 

 that the effect of the two currents balance each other. The 

 resistance to be measured forms part of the fixed coil circuit, and 

 as this resistance increases, the second coil must be moved back to 

 diminish its influence also upon the needle. The moveable coil is 

 acted upon by a constant battery, and the extent of motion im- 

 parted to it, as read off upon a scale, is the measure of the unknown 

 resistance. It is a very convenient instrument, especially for taking 

 great ranges of resistance. 



We now come to the subject of coiling on board ship. The 

 cable on leaving the sheathing-machine, passes into a circular or 

 oval tank, where it is kept covered with water for the convenience 

 of testing, because it is only when under water that it can be well 

 tested. It is next coiled into circular iron tanks on board ship, 

 from the outside to the centre, then passing sharply from the 

 smaller circle to the larger, then back again in a complete spiral, 

 and so on. Formerly cables were coiled into dry ships' holds, but 

 in the case of the Malta and Alexandria cable, water-tight tanks 

 were first suggested. These were not adopted, however, until it 

 had been proved by means of some special electrical tests, which I 

 had provided, that a spontaneous generation of heat was taking 

 place within the mass of the cable, which threatened to melt its 

 insulating covering. The cable had then to be coiled over into a 

 ship, which was provided with water-tight tanks, and ever since, 

 such tanks have been specially provided. 



Tlw Chairman. Is it so in the Great Eastern ? 



Mr. Siemens. Yes, the Great Eastern has been provided with 

 enormous iron tanks to receive the cable. The special means 

 devised to tell the temperature at different points within the mass 

 of the cable, consisted of small coils of insulated copper or platinum 

 wire, encased in iron tubes, which were deposited here and there 

 between the layers of cable, with leading wires from them into the 

 testing-room. The resistance of each coil at standard temperature 



VOL. III. C 



