74 On the Sea-cell as a Possible Source 



Difference of potential between copper of wire and iron = — '146 



„ „ „ iron and sea- water = — *605 



„ „ „ sea-water and zinc = *565 



„ ,j „ zinc and copper of wire = '750 



Total ... ... = -564 



The latter is probably the more reliable authority, and it 

 is certain that the electromotive forces of the more gene- 

 rally known forms of cells, as calculated from Ayrton and 

 Perry's results, agree with those which, from other measure- 

 ments, we know to be correct. 



The first and fourth of these differences of potential are 

 for contacts in air. I do not know what difference, if any, 

 there would be for contacts in sea- water. In the case of the 

 accident, one of these contacts was probably in air; the 

 other may have been either. 



The detonating fuse usually made use of for submarine 

 purposes is that numbered 12 in the table given in the 

 Chatham Instructions in Military Engineering. It consists 

 of a bridge of 3 inch of fine platinum wire, the resistance of 

 which, when cold, is '325 Ohm; at the fusing point, '74 Ohm; 

 and the current sufficient to fire a charge through it, "75 

 Veber. A more sensitive detonator (No. 13) is sometimes 

 used for land service. It is made of finer wire, and its 

 resistance, when cold, is about 108 Ohm ; at the fusing point, 

 about 2'6 Ohms; and the current to fire a charge through it. 

 •32 Veber. 



The wire used in the unfortunate experiment was, as I 

 have said, of No. 16, B.W.G. I have measured the resistance 

 of the length of wire actually used, and found it to be '69 

 Ohm. Its length was 87J yards. According to this, the 

 length of one Ohm resistance was 126*8 yards, or at the rate 

 of about 13f Ohms per mile. This is the value which, for 

 obvious reasons, I have adopted in this paper. 



Assuming that the fuse used was No. 12, and the electro- 

 motive force to be as given by Everett, we have, for the 

 firing current : — 



564 

 Total resistance in circuit = '-— = *752 Ohm, 



*7o 



and this resistance consists solely of the fuse and the wire, 

 that of the cell being nothing. 



If the resistance of the platinum wire did not increase as 

 the temperature of the wire increases, we should have — 



