158 RADIOMETRY. 



and since the temperature of the unexposed junctions is changed an equal 

 amount in the opposite direction the total A*=iXio~ 8 . But a deflec- 

 tion of 1 mm. = 2X io~ 9 C, hence the error is 1 part in 200 under the best 

 theoretical conditions. In practice the temperature sensitiveness will not 

 be so great; it will be shown presently to be of the order 5Xio"~ 6 , whence 

 the Peltier effect would cause an error of 1 part in 500, or 1 mm. in 50 cm., 

 which is as close as one can read such large deflections. 



Since the Joule heat depends upon the square of the current it is negli- 

 gible. Further consideration of the thermopile as an instrument for 

 quantitative measurements will be found below in connection with the 

 bolometer. 



It will be noticed presently that prior to his construction of the iron- 

 constantan thermopile, Rubens had used several very sensitive bolometers, 

 all of which were displaced by the thermopile. For exploring spectra 

 with very narrow lines, the linear bolometer is probably better adapted 

 than the pile which, however, may be covered with a diaphragm, having 

 a narrow slit. For extreme sensitiveness it equals the bolometer, and it. 

 is a noteworthy fact that all the investigations in the extreme infra-red 

 and ultra-violet parts of the spectrum, where the energy is weak, have 

 been accomplished by means of the thermopile. Unless one can build up 

 an elaborate permanent bolometric apparatus in a room not exposed to 

 direct sunlight, the thermopile will give the more reliable readings, as far 

 as the constancy of the zero is concerned. Whether or not the thermopile 

 will give a true reading of the energy falling upon it will depend upon 

 the manner in which it is employed. It requires no particular skill to 

 manipulate, and is easier to protect against temperature changes than 

 is a bolometer with its storage battery. The "drift" in bolometers to be 

 noticed presently was found in the old thermopile when used with a very 

 delicate galvanometer. This was not due to unequal increments of re- 

 sistance as in the bolometer, but to thermoelectric effects at the binding 

 screws, to the connecting wires moving in the earth's magnetic field, and 

 principally to the large heat capacity of the junctions. Most of these 

 disturbances, however, are small and easily avoided in the Rubens type of 

 thermopile. Since the bolometer strips and the balancing coils are of 

 dissimilar material, it is also as subject to thermoelectric disturbances. 



IV. THE RADIOMETER. 



The manner in which an interesting scientific toy can be made to 

 serve a useful purpose is well exemplified in the radiometer of Crookes, 1 

 discovered about 1875. By fastening bits of pith (the one black, the 

 other white) at the ends of a long straw, which was suspended by means 

 of a silk fiber in a long glass tube, he was able to make measurements of 



1 Crookes: Phil. Trans. (II), 166, p. 325 1876. 



