CHAPTER II. 



THE INTERFEROMETER U -TUBE USED AS AN ABSOLUTE ELECTROMETER. 



6. Electrical condenser. Adjusting the U-tube as in figure 5, with the top 

 plates removed so as to admit a metallic disk C, above the mercury M (earthed 

 at E, electrode at a) and parallel to it, CM becomes an absolute electrometer. 

 The disk C is perforated at c, so the component rays L', L" may reach the 

 mercury. Unfortunately this instrument is not very sensitive in any case and 

 is chiefly useful in measuring electrostatic potentials. If p is the electric 

 pressure below the disk (7, charged at potential difference V, and h is the 

 head of mercury resulting, 



( A ) V = d\/%Trp = d \/&irhpg = d 



where d is the distance of C from M, p the density of mercury, and X the wave- 

 length of light when n fringes correspond to V. Hence, if d = o.i cm., 



13.6X981 Xw = 0.3 i7\/w els,. units ; or V= 95\/wVolts. 



In the experiments d was made slightly over 0.38 cm.; but of this, 0.23 cm. 

 were in glass and 0.15 cm. in air. 



The usual experiments with the electroscope were carried out about as con- 

 veniently as with that instrument, with all the data in absolute units. Thus 

 a charged hard-rubber rod near a metallic plate connected with C gave meas- 

 urably up to 40 fringes. 



With larger potentials the images were apt to separate and the fringes van- 

 ish. The range may be varied (d), and steady or alternating potentials are 

 both within reach of the instrument. 



To avoid the effect of specific inductive capacity, a small thin mirror, say 

 i cm. in diameter, may be anchored with loose silk fibers below the hole in the 

 center of the electrode. Otherwise, in the presence of glass plates it would 

 seem that the above equation should be modified to read 



(B) 



and if K K is the specific inductive capacity of the glass, d & and d s the thickness 

 of the layers of air and glass respectively so that d = 



i/K = (d a +d g /K)/d 

 Thus, if K f 6, d g = o.23 cm., d a = o.i5 cm.,i//C = o. 



volts 



where n is the number of fringes. 



It is not, however, obvious that either equation (A) or equation (B) are 

 at once applicable. An investigation of this will be made in 10, showing that 

 equation (A) suffices and that d is to be measured from the top face of the 

 glass plate g to the lower face of the parallel electrode C. 



5 



