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rOWA ACADEMY OF SCIENCE 
Pig. 27. — Apparatus for detecting change in heat conductivity of selenium 
crystals. 
upper part, marked ‘‘heater”. This heat flows through flve 
layers of material to be described immediately, and thence to 
the lower vessel through which tap water flows. There is thus 
maintained a temperature gradient from the top to the bot- 
tom. The five layers are in two parts; the outer, in the form 
of thin washers, and serving as guard rings; and the inner, 
consisting of thin discs. The apparatus is drawn to scale, and 
there are sufficient dimensions shown to make the relative sizes 
clear. The order of the washers, from top to bottom is: copper, 
glass, copper, glass, and copper. The order of the discs from the 
top down is; copper, selenium crystal (fem-like growth, made 
up of hexagonal crystals), copper disc, glass disc, and copper 
disc. Thermo couples of copper and constantan which pass 
through, and are insulated from the outer copper washers, are 
fastened into the three copper discs in the center. As mentioned, 
the purpose of the outer washers was to serve as guard rings, 
so that with a given temperature gradient throughout from top 
to the bottom there would be very little or no lateral heat trans- 
fer. If there is no lateral heat transfer, then we can develop a 
simple equation for the relation of the thermal conductivity of 
the selenium to that of the glass. Assuming that the heat trans- 
ferred passes on from the top to the bottom, we have the formula 
for the relation of the heat conduetivity of the selenium to that 
of the glass^ i 
Kx Ta- T3 
° Tx- Ta 
<See Christiansen, W. Ann. 14, 23, 1881, for derivation of formula. 
