74 Barus — Displacement Interferometer. 



with a horizontal micrometer by which it can be shifted as a 

 whole from k to k' taking the needle with it, k may be elimi- 

 nated. 



The equation may be written in terms of the two forces X 

 (gravitational and electric) 



r Mq A N 4 r 2 A/D 4 V 2 r 2 



■ T^ '- XT 2 L ! 77 11 



^~ t _ i> 2 Kl (l-4A ! /i) s )- D 2 r 



A table was computed for F x and the two forces .5Tmay be 

 found at once. Again, the condition may be practicall}* stated, 

 inasmuch as the values dX/dA may be easily derived. They 

 are for the pendulum Mg/l and for the electric field 



dX 4 V 2 r 2 1 + 12 A 2 /D 2 

 d&~^D 3 l-4A s /i) T 



Equating them, we derive the voltage of transition to be char- 

 acterized by ( T 7 ^), as 



Mg D s 



W 



4 lr 2 



since A = at the origin. Hence above ( T 7 ,) the disk would 

 move on any slight disturbance without interruption from the 

 guard ring, A = 0, to either plate of the condenser ; but within 

 ( F"j) the apparatus is available for accurate measurement of 

 V„ if k is equal to zero by trial. 



8. .Experiments. Cylindrical electrometer ; movable cylin- 

 der within. — Tests of the apparatus described were begun 

 by using a rather heavy cylinder of aluminum, damped with a 

 vane submerged in a cup of oil. The cylinder itself weighed 

 but 6*546 grams ; but air damping was quite inadequate, 

 though the cylinder might easily have been etched to a more 

 appropriate degree of thinness in acid. 



It is therefore necessary to make use of paper cylinders, even 

 if the possibility of securing more nearly accurate values of 

 the diameter of cylinder is the advantage of the metallic 

 appurtenance. 



No trouble was encountered in using a light plane mirror 

 about 1/4 inch in diameter, with axes for adjustment in a bit 

 of cork. To sharpen the ellipses the (yellow) image at the 

 movable mirror should be screened off, leaving the blue image 

 and but two superposed spectra. 



Light gilt paper cylinders, weighing with mirror, etc., only 

 about a gram, were therefore installed. An example (Table 1 

 and fig. 3 a and b) of the many results obtained can only be 

 given here, M being the mass and r the radius of the movable 

 cylinder, and i? the radius of the fixed cylinder. 



