﻿Hence 



The Micro- Azimometer 



4arVo 

 a a < — 3 



385 



which is independent of 7. 



Fig. 3. 



The curve in fig. 3 is plotted from the observed values of 

 6 — d ' in the above experiment. It will be noted that 

 O — 6 O ' is maximum when the potential of the needle is that 

 which corresponds to maximum sensitiveness, which it ought 

 to be, since, when both pairs of quadrants are earthed, we 

 are merely obtaining the deflexion of the needle due to their 

 contact -difference of potential. 



The formula given above for 6 is correct, if it be under- 

 stood that "Vj, V 2 , and V 3 are the potentials of the air in 

 contact with the quadrants and the needle, Q being measured 

 from the position which the needle occupies when Vj, V 2 , V3 

 are all zero, and zero potential being the potential of the air 

 in contact with the case of the instrument. 



Comparing fig. 3 and fig, 2, it will be seen that the contact- 

 difference of potential between the pairs of quadrants was 

 about '01 volt. The contact-difference of potential between 

 two specimens of the same metal might differ by much more 

 than this. 



XXXV. The Micro- Azimometer ; an Instrument for measuring 



small Angular Displacements. By 0, V, BuiiTON, D.Sc* 



1. HPHE chief object of the device here described is to 

 JL increase the sensitiveness of reflecting instruments 

 by optical means, especially when the available reflecting 

 surface is but small. Thus it has been found possible, when 

 the (rectangular) gjH'rture of the mirror measures 3*5 mm. 

 wide X 4t mm. high-, to fitf the azimuth of the mirror with a 



* Communicated by the Author. 



Phil. Mao. S. 6. Vol. 23. No. 135. March 1912. 



D 



