569 



become exti-emely wearisome. But besides all this, great rapidity of 

 the thermopile implies, as is easily conceivable, great stability of the 

 zero, and its indications will therefore be the more certain, the more 

 quickly they are arri\'ed at. 



The common thermopile, as is generally known, is a rather slow 

 instrument. 



In the oiiginal form, devised by Melloni, it consists of a number of 

 metal bars, joined so as to form a bundle, at the terminals of which 

 the two sets of junctions are found. The great heat-capacity of such 

 a system causes the rise in temperature of the exposed junctions to 

 continue a long lime, and thus a considerable delay occurs before 

 equilibrium of temperature is reached. 



The thermopile of Rubens is built up from metal wires, arranged 

 zig-zag wise in the same plane. A slit is adjusted so as to admit 

 the radiation only to the "odd" junctions. The much smaller heat- 

 capacity of this pile is to the advantage of both its rapidity and its 

 sensitiveness. The rapidity, nevertheless, is still comparatively small; 

 in the case of a specimen which 1 examined the thermo-electric 

 current took 6 seconds to reach 99 Vo of its definitive value. 



The improvements, which in recent years the thermopile has 

 undergone, relate to small details of construction only, viz. the number, 

 the dimensions, and the material of the wires. It is true, that some- 

 times the sensitiveness was thus improved, but invariably at the cost 

 of the rapidity. 



The rapidity of a thermopile is determined by the rapidity with 

 which a difference in temperature of the junctions comes to an equi- 

 librium; a process in which not only the heat-capacity of the junctions 

 plays a role, but also, and chiefly, the heat-exchange by conduction, 

 radiation, and convection. 



Heat-exchange also takes place while the instrument is being 

 exposed to the radiation, and will thus have a directly prejudicial 

 effect on the sensitiveness. If therefore this exchange be furthered 

 in order to increase the rapidity, we shall have to expect a dimi- 

 nution of the sensitiveness, unless we arrange so as to compensate 

 for it in some other way. 



Now% for various applications of the thermopile (in particular, when 

 it is employed not for spectral work, but for the measurement of 

 "total radiation") the imrrowness of the exposed surface is of no 

 advantage. A wider surface, which receives a greater part of the 

 radiation, would appear more suitable for the work in hand. 



In the following lines a thermopile will be briefly describedi 



