1894.] establishment of Difference of Potential, fyc. 253 



the possible dangers attending the commercial supply of electrical 

 energy, it was thought desirable to repeat the conditions as nearly 

 as possible, and measure the results obtained. 



As such an experiment could not, for obvious reasons, be satis- 

 factorily carried out in London, it was arranged that it should be 

 tried at Chatham, where the necessary apparatus was available. 



A first experiment was made on the 26th January, 1894. In this a 

 length of 33 ft. 3 in. of 2-in. iron pipe was buried to a depth of 

 18 in. within the salient of one of the demi-bastions at St. Mary's 

 Barracks, Chatham. 



A piece of well insulated 37-strand No. 16 cable was inserted in 

 this pipe so thab the ends of the conductors made good contact 

 with the inside of the pipe at its centre point. 



Over the centre of pipe two 56-lb. weights were placed on the 

 surface of the ground, and at a distance of 4 ft. laterally another 

 pair of 56-lb. weights were similarly placed. 



A 16-kilowatt alternator was connected by means of an under- 

 ground cable laid in stoneware pipes to the piece of cable in the 

 2-in. pipe. 



The other pole of the alternator was connected to a 1^-in. service 

 pipe in connexion with t^e barrack water mains. One end of this 

 service pipe was in connexion with a hydraulic ram set in the ground 

 at a distance of 140 ft. from the piece of buried pipe. 



An 800-volt. Thomson static voltmeter was connected across the 

 terminals of the alternator, and a pair of leads, well insulated, 

 were attached by means of screws to the two pairs of weights situ- 

 ated over the buried pipe. These leads were run back together to 

 the experiment room, and were connected alternately to a 140-volt 

 static voltmeter and a Cardew voltmeter. 



The voltage of the alternator was varied by means of the exciting 

 current which was taken from accumulators. 



The readings on both voltmeters are given in Table I. 



As the earth resistance was rather low, the alternator could not be 

 run up above 400 volts without overheating the armature. 



Fig. 1 is a diagram of the arrangement. 



The results show a potential difference between points 4 ft. apart 

 on the surface of over 20 per cent, of the whole P.D. used to charge 

 the pipe when measured statically, and even when a current of nearly 

 1/10 ampere was drawn from the ground through a Cardew volt- 

 meter the P.D. was more than 10 per cent, of the whole. 



The propinquity of the scarp walls, however, appeared likely to 

 have, to some extent, influenced the direction of the lines of flow of 

 current, and it was determined to repeat it, changing the position of 

 the pipe and extending the tests to a greater distance from the pipe. 

 The second experiment was carried out on the 16th March, 1894. 



