1 64 THE SCIENTIFIC PAPERS OF 



RESISTANCE COIL PEOTECTED BY PLATINUM. The very high 

 degree of heat to which pyrometers have to be raised, renders it 

 necessary to construct them as nearly indestructible by fire as 

 possible, and of a material which is not liable to any permanent 

 change by sudden variations in and elevation of temperature. 

 Platinum is a metal which is well suited for this purpose, in every 

 way, as it does not, when annealed, alter its specific electrical con- 

 ductivity-by the application of heat ; whilst the variation of its 

 measured resistance, due to change of temperature, is sufficiently 

 great to allow of exact readings. But special precautions had to 

 be observed in providing a resistance wire of suitable quality, and 

 in protecting the same from the hot gases of furnaces, which would 

 exercise a chemical action upon it. 



The pyrometer coil which I prefer is made of fine platinum 

 wire of O'0 1 inch diameter, the resistance of which averages 3*6 

 units per yard of length. This wire is coiled upon a cylinder of 

 hard baked pipe-clay in which a double threaded helical groove is 

 formed, to prevent the convolutions from coming into contact, with 

 each other. The form of pipe-clay cylinder is shown inFig. 7, Plate 13. 

 At each end of the spiral portion, BB, it is provided with a ring- 

 formed projecting rim c and c', the purpose of which is to keep the 

 cylinder in place when it is inserted in the outer metal case, and 

 to prevent the possibility of contact between the case an 1 the 

 platinum wire. Through the lower ring c', are two sma 1 holes, 

 bb', and through the upper portion two others aa'. Ths purpose 

 of the upper holes, aa', is for passing the ends of the platinum 

 wires through, before connecting them with the leading wires. 

 From these two holes, downwards, platinum wires are coiled in 

 parallel convolutions round the cylinder to the bottom, where 

 they are passed separately through the holes bb'. Here, they 

 are twisted, and, by preference, fused together by means of 

 an cxy-hydrogen blow-pipe. At this end, also, the effective 

 length and resistance of the platinum wire can be adjusted, 

 which is accomplished by forming a return loop of the wire, 

 and providing a connecting screw-link of platinum, L, by 

 which any portion of the loop can be cut off from the electric 

 circuit. 



The pipe-clay cylinder is inserted in the lower portion, A A, of the 

 protecting case, shown in Fig. 8, Plate 13. This part of the case is 



