374 THE ELECTRIC TELEGRAPH. 



cidedly affected- a length of 162 knots was cut into three 

 parts, and coiled into a dry tank, where its electrical con- 

 ditions were from time to time narrowly examined. Its 

 copper resistance had risen to a degree which indicated a 

 temperature of 80 Fah., while the air and water in the 

 neighbourhood were not above 57 Fah. 



Considerable anxiety was, of course, manifested on the 

 occasion of this unlooked-for mishap, and Dr. Miller of 

 King's College undertook for the Board of Trade to inquire 

 into and report upon the subject. In his Report he says 

 that, " These heating effects appear to have been due, not 

 to any permanent chemical change, either in the composition 

 of the insulating coating of the gutta-percha, nor in that of 

 the serving of hemp and tow ; but were owing simply to the 

 effects of oxidation upon the iron at the ordinary summer tem- 

 perature of the air, produced by the moistening of the cable 

 with the water of the river, the slightly brackish nature 

 of which increased the effect. " In support of this opinion 

 a quantity of iron filings was placed in a wooden box, a 

 foot deep, and water at 40 F. poured over them. In a 

 few hours the temperature of the mass had gone up con- 

 siderably, and, after a day and a half, reached 100 F. The 

 rate at which this increase of temperature went on was 

 found to depend upon the frequency with which the filings 

 were stirred and watered. 



In order to measure the degree of self-heating, and to 

 ascertain at all times the temperature of a cable when coiled 

 in the tanks, both in the yard and on board the ship, 

 Mr. "William Siemens constructed a resistance- thermometer. 



This " resistance^ thermometer" consists of a coil of fine 

 copper or other pure metal wire, whose resistance at Cels. 

 is .1 00 or 1,000 units. The per-centage variation of the 

 resistance of pure metals between certain limits of tempera- 

 ture being known, by measuring the -resistance of the coil 

 at any moment, its temperature can be calculated, and that 

 of the surrounding medium concluded. A more useful and 

 unerring measure of temperature than this resistance-ther- 

 mometer does not exist. One of the chief advantages which 



