570 



The necessity of sharp adjustment at a definite spot is very evident 

 here. If e.g. we have gauged the thermopile at adjustment 16, and 

 if we use tlie pile later at adjustment 17, it follows from the table 

 that errors can t)e made of the order of 50"/\,, at least when again 

 beams of 0.2 mm. are used. 



Hence a displacement of only 1 mm. can already have a great 

 influence. It is also clear that the dev^iation is greatly dei)endent oji 

 the width of the used beam of light. 



When we consider e.g. 2 beams that send the same quantity of 

 energy to the thermopile per second, the centres of which coincide 

 in the maximum of sensitivity of the pile, but the widths of which 

 differ, and may be successively 1 and 3 mm., it follows from the 

 table that in the case of the narrow beam a deviation is obtained 

 about 207o greater. In this calculation the mean values of the sensi- 

 tivities have been used. 



It follows further from the graphical representation that the ther- 

 mopile has been made very uniform. The maximum sensitivities all 

 lie at 17,5 mm. This uniform and sharp appearance of the sensitivity 

 maximum enables us nou to use this as criteiion of adjustment. 



The procedure in this is as follows : The function of width is 

 determined with the beam to be used, as has been done above ; 

 from this it is possible to determine accurately with what adjustment 

 the beam can show the greatest deviation. Then we adjust the tliei-mo- 

 pile in the required position, after which we carry out the desired 

 measurements. 



This operation, which can take place quickly, is carried out 

 before the gauging as well as before the use of the thermopile.^) 



In this way the difliculties with the adjustment have been solved. 



With regard to the width of the beams it may be observed that 

 in these experiments use was made of a blackened platinum plate 

 1 mm. wide, which was at a distance of 1 mm. from the thermopile. 



This plate can now be used in a simple way to make also the 

 beams of light about 1 mm. wide. 



In the gauging the used beam of light was obtained by cutting 

 out by means of a screen from a larger parallel beam of light origin- 

 ating from a Nernstlamp. The screen is provided with screen doors, 

 so that the beam to be used can be made broader and smaller. 



i) With regard to this adjustment Dr Moll informed me lately, that for relative 

 measm'ements he also works with the greatest sensitiveness, by rotating the thermo- 

 pile to and fro. As a very accurate adjustment is necessary for the gauging, I 

 prefer to determine for this the function of width in order to determine the maxi- 

 mum adjustment graphically from the ditïerenl points that have been found. 



