4:66 C. C. Hutchins — Measurement of Radiation. 



Art. LTI. — A new instrument for the measurement of Radia- 

 tion ; by C. C. Hutchins. 



The difficulties which attend the use of the thermopile as an 

 accurate measurer of radiations, are familiar to all who have 

 had any experience with that instrument. The slowness of its 

 indications, and the long time required for it to return to zero, 

 are defects which entirely unfit it for many delicate experi- 

 ments. 



It occurred to the writer that sensitiveness to radiation might 

 as well be secured by employing a very thin thermal -j unction 

 with some condensing arrangement, as by the use of several 

 pairs of stout bars, as in the ordinary thermo-pile ; for the thin 

 junction would be' heated to a much higher temperature than a 

 thick one by a given quantity of heat, and have the great ad- 

 vantage of quickly parting with its heat and returning to the 

 temperature of the surrounding atmosphere. 



The instrument is constructed upon these principles as fol- 

 lows : A tube of vulcanite ten inches long, two and a half 

 inches in diameter, is stopped near the middle by a plug of 

 wood. The tube is made separable, and this plug serves to 

 unite its two halves as well as to support the working parts. 

 Through the plug pass two small copper rods projecting about 

 an inch above the plug toward the front of the instrument, and 

 passing out through its back, where they serve to attach wires 

 extending to a galvanometer. 



E, tube of vulcanite; C, plug of wood; m, n, copper rods; A, thermal junction ; 

 B, concave mirror ; D, stop. / 



The thermal junction is made by uniting with hard solder a 

 bit of watch-spring and a bit of flattened copper wire. The 

 whole is then worked to a ribbon l mm wide, *03 mm thick and 

 25 mm long. The two ends of this ribbon are then soldered to 

 the two copper rods so that the junction may be midway be- 

 tween them. 



A concave mirror of glass, silvered upon first surface, is so 

 secured upon the plug that the junction is exactly at its focus. 

 The front of the tube is provided with an opening of any con- 

 venient size, and stops to limit the diameter of the entering ray. 



