1 70 RADIOMETRY. 



Comparison of Sensitiveness of Various Bolometer-Galvanometer 



Combinations. 



The most important data on sensitive radiation meters, and particu- 

 larly that relating to bolometers, is given in table VII. It will be noticed 

 that the thermopile is as sensitive as the bolometer. The sensitiveness of 

 the radiometer is obtained by comparing it with the bolometer. Although 

 the radiometer is no doubt less efficient than the bolometer, it probably 

 absorbs as much of the incident energy. Since the radiometer deflections 

 were larger, per unit area, than the bolometer deflections, it is safe to 

 assume that the radiometer was just as sensitive as, if not more so than, 

 the bolometer. The long period is of course a serious objection in certain 

 classes of work. In table VII it will be noticed that the high temperature 

 sensitiveness of the various instruments has been attained by the use of 

 a highly sensitive, long period, galvanometer, by using a large bolometer 

 current and by placing the scale at a great distance from the galvanometer. 

 Assuming that the sensitiveness is proportional to the square of the period 

 (undamped) for a scale at 1 m. and a bolometer current of 0.04 ampere, 

 it will be noticed from column 10, table VII, that the temperature sensitive- 

 ness of the various instruments falls in two groups. To the first group 

 belong the earlier instruments of Rubens, of Snow, and of Paschen, with 

 a sensitiveness of about 5X io -5 per millimeter deflection. To the second 

 group belongs a more sensitive combination of Paschen's, and the writer's 

 instrument, in which 1 mm. deflection corresponds to a rise in temperature 

 of iiXio~ 8 and oXio -6 C., respectively. In other words, the instru- 

 ments of the latter group have the same sensitiveness, and any increase in 

 the same is to be attained by increasing the scale distance; the bolometer 

 current of 0.04 amp. is about the limit for accuracy. The sensitiveness 

 of the writer's instruments could have been further increased by length- 

 ening the scale distance to 2.5 m., when the temperature sensitiveness 

 would have been 3.6Xio -6 , and by doubling the galvanometer period, 

 when the sensitiveness would have been qXio -7 against Paschen's iX 

 io -8 . Such a computation is, of course, illusory on account of damping 

 in the galvanometer. On actual trial (but not for the magnet system 

 quoted) for a full period of 30 seconds the sensitiveness of the galvan- 

 ometer was i=7Xio -11 ampere. 



Comparison of a Bolometer with a Thermopile. 



The efficiency of the bolometer and the thermopile is reduced by losses 

 due to reflection and radiation from the receiving surface, and by heat 

 conduction to the unexposed parts. The loss of energy in the thermopile 

 due to the Peltier effect has been considered in discussing that instrument. 

 The loss of energy due to reflection is about 4 per cent (Kurlbaum, loc. 

 cit.). Assuming the bolometer to be made of platinum 0.5X0.002 mm. 

 cross-section, and the thermopile of 20 junctions of iron and constantan 



