COBLENTZ: NEW DESIGNS OF RADIOMETERS 475 



circuit to the galvanometer will depend upon the relative change 

 in voltage of the two circuits when the receiver is exposed to 

 radiation. If the error due to shunting is too great when the 

 two circuits are joined to the same binding posts, the bolometer 

 current can be passed through one galvanometer coil and the 

 thermopile current through another coil. 



The simplest and probably the most useful arrangement is a 

 bolometer consisting of two branches of thin narrow strips of 

 platinum close back of which is placed a thermocouple. In 

 measuring the heat from stars a gain of only 50 per cent in sen- 

 sitivity is worth considering. 



The multiple thermocouple receiver. The use of two thermo- 

 couples, joined in series, with the receivers one back of the other, 

 has not yet proved to be so efficient, because of the greater heat 

 capacity of the thermocouple receiver used as compared with a 

 bolometer. The comparison of this combination with the two 

 preceding instruments, and with a single thermocouple (or bo- 

 lometer) in the focus of a spherical inclosure, in which all the 

 parts are reduced to the dimensions which would be used in 

 measuring stellar radiations, is in progress. 



In conclusion it may be added that as a result of the writer's 

 previous measurements of stellar radiation 4 the conclusion was 

 arrived at that, in order to do much successful work in stellar 

 radiometry, it will be necessary to have a 100-fold greater sensi- 

 tivity than that previously employed. This gain in sensitivity 

 was to be attained by increasing the light-gathering power of 

 the telescope 5 times, the sensitivity of the galvanometer 10 

 times, and the radiometer sensitivity 2 times. In a paper 5 just 

 published data are given showing an increase of more than 10 

 times in the galvanometer sensitivity, while the present paper 

 indicates the way to double the radiometer sensitivity. Appar- 

 ently then it remains to find a suitable mirror and funds to 

 operate it. 



* Bull. Bur. Stds., 11: 613. 1914. 

 6 Bull. Bur. Stds., 13: 423. 1916. 



