1871.] of Testing a Telegraph Earth. 185 



the surface of the ground, and 4 feet from the centres of A and B. 

 Eesistance of plate A = 8-95 S. U 

 „ B = 6*2 „ „ 

 ,, C = 11-77 „ „ 

 The values are somewhat higher than those previously obtained, 

 but this I think is due to the ground being drier than before. 



Experiment 3. Plates A and B remained as before, but B was 

 placed horizontally, with its centre only 6 inches below the surface. 

 Eesistance of plate A = 9*44 S. U 

 » B = 17-49 „ „ 

 „ C = 11-92 „ „ 

 so that the resistance of B is nearly three times as great as it was 

 before. 



Experiment 4. All the plates in the same position as in the last 

 experiment, but B was well surrounded with a layer of charcoal. 

 Resistance of plate A = 9*39 S. U 

 „ B = 17-04 „ „ 

 ,, ,, \j = 11*99 ,, ,, 



these results are very nearly the same as those obtained in experi- 

 ment 3. 



As a general rule, it may be said, that the lower an earth 

 plate is buried the less is its resistance, or the better is it for tele- 

 graphic purposes. This rule, however, must be used with caution, 

 because it assumes that the ground is of the same conducting 

 quality at the different depths. 



For example at the Jubbulpore office in order to obtain a good 

 " earth" a large copper earth plate 5 feet square was buried ver- 

 tically 17 feet deep. On my passing through Jubbulpore I tested 

 this earth plate and found that it had a resistance of 50 S. U. On 

 having the plate dug up I found that the high resistance was due 

 to the plate having been buried in solid sandstone. I afterwards 

 had the plate buried horizontally in the upper stratum of the soil 

 and well surrounded with charcoal, and now the resistance was only 

 20 S. U, or less than one half of what it was before. 



At Eaneegunge Mr. Schwendler found that the earth plate there 

 never had more than 0-5 S. U resistance. This very low resistance 

 was probably due to the coal which exists there from 2 to 4 feet 

 below the surface and in some places is actually exposed. 



