76 Intelligence and Miscellaneous Articles. 



With alcohol there is no such change in the sign of the charge. — 

 Wiedemann's Annalen, August 1887. 



A NEW INSTRUMENT FOR THE MEASUREMENT OF RADIATION. 

 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 indica- 

 tions, and the long time required for it to return to zero, are 

 defects which entirely unfit it for many delicate experiments. 



It occurred to the writer that sensitiveness to radiation might as 

 well be secured by employing a very thin thermal junction with 

 some condensing arrangement, as by the use of several pain of 

 stout bars as in the ordinary thermopile ; 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 advantage of quickly 

 parting with its heat and returning to the temperature of the 

 surrounding atmosphere. 



The instrument is constructed upon these principles as follows : — 

 A tube of vulcanite ten inches long, two and a half inches in dia- 

 meter, 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 towards 

 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 1 millim. wide, *03 millim. thick and 

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

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

 them. 



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

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

 The front of the tube is provided \\ itli an opening of any convenient 

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



The accompanying sketch will make the details clear. Its 

 working has been very satisfactory. It requires no longer to 

 return to zero than for the galvanometer-needle to come to rest, 

 and is correspondingly rapid and dead-heat in its action. It is 

 much more sensitive than a thermopile of tin- same exposed area. 



