Page 113 - My FlipBook

pearance. To both ends of these two rods are fused platinum lead wires
for conveying the current to the "glower." These glowers have the
peculiar property of changing in resistance to conductivity of the cur-
rent according to the temperature to which they are exposed. To illus-
trate this point more clearly, the "Nernst glower" is a perfect non-con-
ductor when cold, but when heated it becomes a conductor of the elec-
tric current, and its conductivity increases as the temperature to which it
is exposed increases. By placing one or more of these glowers in the

Fig. 123.

muffle (Fig. 122) and connecting them in series with a milli-volta-
meter which is also in series with a stable low voltage circuit, the heat
of the muffle is recorded. The amount of current that is required to
actuate the instrument is very small : hence a stable unvarying current
supply is secured with three ordinary dry cells. These cells will maintain
this form of pyrometer in an accurate condition for a period of at least
one year. In connection with this pyrometer there is a means by which
the accuracy of the instrument may be instantly tested before each firing.
There is interposed in the battery and instrument circuit a resistance coil
operated by a small push button switch. By merely pressing on this

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