Bauhaus-Universität Weimar

A Thermopile for Measuring Radiation. 259 
The new construction has the great advantage that it is not necessary to screen 
off the passive junctions ; indeed the elements can be exposed to radiation along 
their full length. It is therefore possible to arrange the elements areally, and to- 
construct surface-thermopiles. 
Fig. 3 shows the grouping of 80 elements within a circle of 2 cm. diameter. 
Fig. 3. 
Such a thermopile has a resistance of about 50 ohms, and yields about 75 microvolts 
for the radiation of a candle at 1 metre, when closed by a plate of rocksaït. 
The elements may also be arranged on a smaller surface ; this increases the 
sensitivity per unit of area. Fig. 4 shows a type of construction, which in our 
investigations has largely fulfilled the requirements. This thermopile is closed by 
a window of fluorite having a circular opening of 6 mm., behind which 18 elements 
are arranged. Its resistance is about 25 ohms, and a candle at 1 metre generates- 
Fig. 4. 
about 13 microvolts. By providing the thermopile with a conical reflector the sensi¬ 
tivity may be largely increased. 
Both thermopiles are equally quick in response. If they are connected to a. 
sufficiently rapid galvanometer (adjusted so as to have its indications dead beat),, 
and radiation is periodically admitted and intercepted, the spot comes to a perfect 
standstill within 2 seconds.


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