158 THE SCIENTIFIC PAPERS OF 



PART SECOND. 



ON MEASURING TEMPERATURES, INCLUDING FURNACE 

 TEMPERATURES, BY ELECTRICAL EESISTANCE. 



IN the early days of submarine telegraphs, it frequently happened 

 that the insulated conductor, which had tested well at the cable 

 works, proved faulty after the cable had been submerged, and, 

 upon examining such faulty cable, the metallic conductor was 

 found to have sunk through the gutta-percha covering, an effect 

 which could not be satisfactorily accounted for by accidental 

 causes, such as may arise in joining wires during the process of 

 manufacture ; whereas the effect of heat of an intensity of 

 at least 38 Centigrade, or of sufficient intensity to soften or 

 melt the gutta-percha covering of the cable, was generally 

 traceable. 



In 1860, when professionally engaged on behalf of Her Majesty's 

 Government in superintending the examination of the electrical 

 condition of the Malta and Alexandria Telegraph Cable, during 

 its manufacture and submersion, it appeared to me that heat, as 

 revealed by its disastrous effects, might be spontaneously gene- 

 rated within a large mass of cable, either when coiled up at the 

 works or on board ship, owing to the influence of the moist 

 hemp and iron wire composing its armature. In considering the 

 means by which such rise of temperature within the mass might 

 be observed, my attention was directed towards that property of 

 metallic conductors of offering, in a rising temperature, an in- 

 creasing resistance to an electrical current, to which attention 

 has been drawn in the first part of this lecture. 



Now, an instrument constructed on the principle of the in- 

 crease of electrical resistance with rise of temperature, would 

 possess the obvious advantage that the metallic conductor under 

 observation might be at some distance from the observing in- 

 strument, and need not be disturbed for making observations. 

 Accordingly, I prepared coils of copper wire insulated with silk, 

 whose electrical resistance having been ascertained and adjusted, 

 were enclosed in iron tubes, with the ends hermetically sealed, 

 but allowing thick insulated leading-wires to pass outward. 



