244 Professor Fleming [June 5, 



perature of the air. We may also compare the resistances of these 

 two similar iron coils, when one is placed in boiling liquid air and 

 the other in boiling water. The resistances, instead of being in the 

 ratio of one to one, are now in the ratio of one to twelve. Again, if 

 we take two wires, one of pure iron and one of pure copper, of exactly- 

 equal length and equal section, we find that at ordinary temperatures 

 (15° C.) the iron wire has about six times the resistance of the coj)per : 

 but if we cool down the iron wire in liquid air to — 186° C, still 

 keeping the copper coil at the ordinary temperature (15° C), we now 

 find that the iron coil has actually become a much better conductor 

 (about 30 per cent, better) than the copper.* On the other hand, if 

 we examine the behaviour of this coil of German silver, which is a 

 copper-zinc-nickel alloy, or of this platinum-silver coil, we find that 

 the cooling down through 200° has a comparatively small effect upon 

 its electrical resistance. We thus see that whilst pure metals have 

 their electrical resistance immensely decreased by cooling to the 

 temperature of liquid air, alloys generally do not experience anything 

 like so great a change. 



A word or two must next be said on the manner in which 

 we have represented graphically all the results of our experiments. 

 We desired to delineate lines on a chart so as to express the 

 change in specific resistance of all our metals and alloys in terms of 

 temperature ; and the question then arises, how was the temperature 

 measured ? You already know that an ordinary thermometer, 

 whether mercury, alcohol, or air, would be useless to measure tem- 

 peratures at which even air liquefies under ordinary pressures. 



The employment of the constant pressure hydrogen thermometer 

 with reduced pressure would have given us temperature readings very 

 approximately those of the absolute thermodynamic scale, but the 

 experimental difficulties of its use would have been enormous. We 

 preferred to use the platinum resistance thermometer, and to express 

 our temperatures in platinum degrees as follows : — Our experience 

 has shown us that a pure soft annealed platinum wire may be cooled 

 as often as necessary to the lowest attainable temperatures, and yet 

 will always have the same resistance when measured again at other 

 constant temperatures. Availing oui'selves of this fact, we have used 

 in all this work a low temperature platinum thermometer made in 

 the following way : — A well-annealed platinum wire is made into a 

 resistance coil, as already described. Its resistance is carefully 

 measured at the temperature of boiling water, 100° C, and melting 

 ice, 0° C. From these measurements we construct a scale of tempera- 

 ture as follows : — A horizontal line A E (see Fig. 5) is taken on 

 which to mark off temperature, and any two points A and B are 

 taken on this line and the length A B divided into one hundred 

 equal parts. At these points B and A perpendiculars are set up 



* The exact resistances of the coils used for the experiment were as follows : 

 Iron at 16° C. = 7*003 ohms, and reduces to 0*711 ohms; at - 186° C. copper 

 at 16° = 1*169 ohms, reduces to 0-2033 at - 186° C. 



