Research on some Standards of Light. 



483 



In order to reduce the amount of heat dissipated we must place the 

 platinum in an enclosure at nearly the same temperature as the metal 

 itself, take the observations through a relatively small opening, and 

 make the thermal insulation of the mass as perfect as possible. In the 

 above formulae, L being a function of the volume and D a function of 

 the surface, we may, for any given circumstances, increase the time t 

 by increasing the total volume of the metal. 



But it now becomes necessary to take into account the thermal con- 

 ductivity of the metal. Strictly speaking, any given portion of the 

 surface can only be at the standard temperature t s for the very small 

 fraction of a second during which the solid film is forming ; later on 

 the surface is at some lower temperature (t s - n) where n is the 

 temperature interval which is required to cause the heat to flow from 

 the parts of the mass where the process of solidification is actually in 

 progress to the surface at the same rate as it is there being dissipated. 

 Thus by increasing the mass of the metal we cannot indefinitely in- 

 crease the time available for the photometric observations. Unless 

 the rate at which the heat is being dissipated at the surface under 

 observation is made small, the temper at ure will not remain constant 

 during the process of solidification, but will fall more and more rapidly 

 as a larger proportion of the mass becomes solid. 



On the Fusion of Platinum by an Electric Current. 



From the considerations that have just been enumerated, the ad- 

 vantages of an electric method of heating the metal are at once 

 apparent. 



In the preliminary experiments a number of different forms of 

 electric furnace were tried. In some cases the electric arc was used, 

 in others the crucible was surrounded with a layer of graphite through 

 which the heating current was passed. It is well known that at high 

 temperatures platinum combines with carbon forming a carbide, and 

 thus the use of graphite crucibles was out of the question. 



It is unnecessary to give further details as the experiments were 

 not successful. No material could be found that would resist the 

 high temperature of the arc and the chemical action of the incandescent 

 carbon, and at the same time not affect the purity of the platinum. 



It was then decided to fuse the platinum by passing the current 

 directly through the metal itself. The apparatus shown in fig. 5 was 

 designed for this purpose. Some forty large secondary cells were, by 

 aid of the mercury switch, connected in parallel on to two " 'bus-bars." 

 The current was carried by massive copper leads to the terminal Ti, 

 and from there by a one square inch bar to the clip C 1 passing through 

 the platinum bar A to the clip C 2 . R is a U-shaped mercury trough 

 serving as a variable resistance. The copper short-circuiting piece D 



