1 76 RADIOMETRY. 



was found that the radiometer is not selective in its action, and, hence, 

 that it can be used for measuring ultra-violet radiation. The main objec- 

 tion to the use of a radiometer is its long period; but, since it is easily 

 shielded from temperature changes, and since it is not subject to magnetic 

 perturbations, this long period is of minor importance so long as we are 

 dealing with a constant source of radiation. In spectrum energy work its 

 usefulness is limited to the region in which the window is transparent 

 to 20 // and from 40 to 60 [i by using quartz. The fact that the deflections 

 of the radiometer can not be obtained in absolute measure is a minor 

 objection, since in but few cases (thus far at least) has it been necessary 

 to thus obtain the deflections. The action of a radiometer is somewhat 

 analogous to a photographic plate, in that it will detect weak radiation, 

 provided one can wait for it, and, on account of its great steadiness, is, 

 of all the instruments considered, probably the best adapted in searching 

 for infra-red fluorescence. 



A bolometer installation is so distributed that it is difficult to shield 

 from temperature changes. In spite of its small heat capacity the bo- 

 lometer has a "drift" due to a slow and unequal warming of the strips. 

 Air-currents which result from the hot bolometer strips also cause a varia- 

 tion in the deflections of the auxiliary galvanometer. Nevertheless, despite 

 these defects it is the quickest acting of the four instruments considered, 

 and is the best adapted for registering the energy radiated from a rapidly 

 changing source. For precision work it is necessary to keep the bolometer 

 balanced to less than 1 cm. deflection. 



The auxiliary galvanometer is the main source of weakness in measur- 

 ing radiant energy, and in places subject to great magnetic perturbations 

 a period greater than 5 seconds, single swing is to be avoided. Hence, 

 although a greater sensitiveness is possible, the working sensibility of the 

 various galvanometers studied is of the order of i=2Xio -10 ampere per 

 millimeter deflection on a scale at 1 m. Under these conditions the various 

 bolometers used were (as a fair estimate of the recorded data) sensitive to 

 a temperature difference of 4Xio -5 to 5Xio~ 6 per mm. deflection, on a 

 scale of 1 m. The galvanometer sensibility was found to be closely pro- 

 portional to the period. 



A direct comparison of the thermopile and the bolometer shows that 

 there is little preference, other than a personal one, in these two instru- 

 ments. 



The use of a rotating sectored disk for reducing the intensity of the 

 source is liable to introduce errors, which must be taken into account. 



