Ixxxvi 



another 4,000 " ohms." Before a cable was laid down, it was tested to 

 find the resistance, and discover any leakage of the current, and these 

 tests were kept for future reference. A good cable would improve in 

 its insulation after being down a short time, and should show about the 

 same amount of resistance and insulation as when out of the water. A 

 certain length of cable with a conductor of a certain size should give a 

 certain number of " ohms" of resistance to the mile. It would show no 

 current so long as the further end of the cable was insulated, but if 

 there were any leakage a portion of the current would come back to the 

 station, and the resistance would, of course, be proportionately decreased. 

 The lecturer exhibited a galvanometer, used to detect the presence of 

 weak currents of electricity, and of so delicate a character, that it 

 would be possible by it to read signals transmitted through the Atlantic 

 cable from a battery, the size of an ordinary gun cap. Having described, 

 in detail, the construction of the galvanometer, which consisted mainly 

 of a very light and small glass mirror, having attached to the back of it 

 a magnet, made of a piece of watch spring, and the whole suspended 

 by a fibre of raw silk between two coils of wire ; he then proceeded to 

 give a practical illustration of the manner in which the locality of a break 

 in the cable may be determined. Having a coil of insulated wire some 

 1,500 yards in length submersced in a bath of salt water, he connected 

 it with the galvanometer and box of resistance coils, technically called a 

 *'Wheatstone bridge," and showed the resistance of the cable to be 293 

 "ohms." Some of the insulating covering was now removed to make a 

 fault in the cable, and tests were again applied from either end of the 

 cable, the resistance being in the one case 273, and in the other, 23 

 "ohms," making a total of 296, which showed the fault to be making 

 nearly '* dead earth," as nearly the whole of the current passed through 

 it to earth. It was therefore apparent, supposingan"ohm"in this instance 

 to represent a mile of cable, that the fault had occurred at a distance of 

 about 273 miles from one station, or 23 miles from the other. Lastly, 

 Mr. Henry described minutely the various processes required to secure 

 an accurate test, and the elaborate calculations that have to be made to 

 ensure any degree of certainty. 



In reply to the Chairman, Mr. Henry said it was much easier to 

 determine the locality of a break in a cable than on land lines, the 

 insulation in the latter case being so much more imperfect, rendering a 

 complete test very difficult. He had recently made a test on the line 

 between Hobart and Launceston, where a break had occurred, but he 

 proved to be several miles out in his calculation. 



The Hon. Dr. Agxew proposed that the best thanks of the meeting be 

 passed to Mr. Henry for the extremely interesting lecture he had given. 

 The subject tvas an intricate one, and required a certain amount of 

 preliminary knowledge before it was possible to grasp all its details as 

 presented in the course of a single lecture. He was sure, however, every- 

 one present felt obliged for the practical and scientific exposition of the 

 matter with which Mr. Henry had favoured them. 



The Chairman, in putting the vote, said the subject was of more than 

 scientific interest, for it was one in which all people must take an interest. 

 They liked to have the news of the world at their breakfast table, and 

 were disappointed if they did not get it ; and they were therefore deeply 

 interested' in the question of how a break may be found, and how it may 

 then be mended. 



The vote was carried with acclamation, and Mr. Henry briefly returned 

 thanks. 



A vote of thanks was passed to those who had forwarded donations to 

 the library and museums, and the meeting terminated . 



