468 



Mr. W. H. Preece. On the 



[Apr. 3, 



and convection, the temperature becomes constant. But the heat 

 lost varies with the surface, and the smaller the surface the less the 

 loss, and therefore the temperature will vary with the diameter. 

 Hence as — 



v2 



and also Hoc d, 



C 2 — " 



therefore ■ — =d, or, C — dVd 



The results of the foregoing experiments on the current required 

 to fuse wires of different diameters would appear to contradict this 

 law, and to show that the current varies as the diameter, but there 

 are several elements in the method of making the measurements 

 which tend to modify the general law. Platinum wires — especially 

 those of small diameter — are liable to flaws which practically reduce 

 their effective diameter to a great extent ; also the larger wires from 

 their greater weight necessarily tend to part asunder at a lower tem- 

 perature than those which are lighter and on which the strain is less. 



3. In thermometric measurements there are only two fiducial 

 points usually employed, viz., the freezing point, and the boiling- 

 point, but there are two other points equally well marked, though 

 perhaps not quite so accurately fixed. These are the points of self- 

 luminosity and of fusion. The first point has been determined by 



Daniell to be ^526°'^' (3 }' an( ^ ^ Draper at j^25° }' an( ^ ^ 

 has been shown by the latter observer that this is true for all 

 substances. 



I was anxious to discover whether the law before stated would 

 apply in the case of the production of the lowest visible red radiations 

 by currents. I therefore took several wires of different materials, and 

 of different diameters, but all of the same length, and by observing 

 them in a carefully constructed dark chamber, determined the 

 currents that produced self -luminosity, these currents being measured 

 by finding the difference of potential at the ends of a thick German 

 silver wire (R) whose resistance was *0157 w , inserted in the same 

 circuit. The results of the experiments, which were very carefully 

 made, are given in the following tables : — 



