4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 



switch H and the blocks C, C\ D, Z)\ the electrical" current for heat- 

 ing the strips A, B, divides into two. Appropriate* resistances are 

 placed in the two circuits, so that although the strip B is ten times 

 as thick as the strip A, and has a correspondingly smaller electrical 

 resistance, compensation is provided by means of the said electrical 

 resistances so that the current divides in the proper proportion to 

 heat the strip A at exactly the same rate as strip B. 



The two U-shaped thermo-elements, /, 7^, are arranged in series, 

 with their warm and cold junctions respectively attached by thin 

 waxed paper to the back of the strips A and B, so that the difference 

 of temperature (if any) between the strips A and B is indicated by 

 means of a galvanometer connected into circuit with them by means 

 of a flexible conductor (not shown in the figure) which enters the 

 instrument by means of the tube / shown in figure 4. We employ 

 tellurium-platinum for the thermo-elements on account of the great 

 thermo-electric power of this combination, the non-corrosion of plati- 

 num by melting tellurium, and the small thermal conductivity of 

 tellurium. The difficulty of forming tellurium into the U-shaped 

 elements shown was at first considerable, but was overcome after 

 some practice. 



The principle of operation of the instrument may now be under- 

 stood. Radiation falling simultaneously upon the strips A and B 

 communicates to each the same quantity of heat ; but the rise of 

 temperature after a steady state is produced thereby is different in 

 the two because the strip B is ten times as thick as the strip A, and 

 so its thermal conductivity to the ends is greater. Hence a deflection 

 of the galvanometer occurs. This deflection is balanced, after again 

 shading the strips, by means of an electric current divided between 

 the strips A and B so as to produce equal heating effects in each. By 

 suitable adjustment the deflection of the galvanometer which was 

 produced by the absorption of radiation is reproduced by the heating 

 of the electric current. In these circumstances the energy of the 

 electric current transformed into heat in either strip is equal to the 

 energy of the radiation absorbed by either strip. The instrument is 

 primarily designed to measure radiation on a horizontal surface, but 

 it can be used in any position. 



The remaining details of the instrument will be easily understood. 

 K is an optically figured hollow hemispherical screen of ultra-violet 

 crown glass 25 mm. in diameter and 2 mm. thick, whose purpose 

 is to admit direct or scattered solar radiation, but to prevent the 

 exchange of long wave-length radiation between the manganin 

 strips and the sky. A nickel-plated shutter, L, is provided for 



