Vol.. 7, 1921 PHYSICS: C. BARUS 243 
around a, a' and d, d r and admitting of small displacements along d. At 
the other end of each rod is a flat vertical plate which is received in a 
fissure at the top of the corresponding hard rubber post k, k' and clamped. 
This gives a horizontal axis at right angles to the preceding. The lower 
ends of the posts k, k' are suitably clamped to the tubes t, t\ attached to 
the body B of the electrometer. Here further motion along the tubes t, t' 
and rotation around them is possible. In this way it is not difficult to 
place C, C symmetrically above the mercury pools and parallel to their 
upper faces, for experimental purposes. It is not sufficient, however, if 
precision is required. 
Figure 1 shows the Mascart key below on the right, which consists of 
the elastic brass strips I, the earthed cross bar n above them and the 
cross bar q below. Thus the whole U-tube is earthed when not in use. 
The bar q is connected by wires with the brush A of the electrophorus 
shown on the left, so that when I or V are depressed into contact with q f 
C or C' t respectively, receives a positive charge while the other electrode 
and the mercury is earthed. This affords a very satisfactory means of 
commutation; for since AV=C-\ l n = C'^n', the electrodes are so ad- 
justed that C and C are nearly equal. 
, Tests were made with this apparatus and a known AV = 173 volts. 
For example, the scale readings in the ocular on commutation were % = 34, 
%' = 17 so that {%-%') = .7 (n + n'), as the fringe breadth was .7 scale 
parts. Thus AV = A V24^ or A = 35.3 volts per fringe, initially. With 
large fringes and under quiet surroundings 3 or 4 volts could have been 
detected. 
The upper face of the electrophorus p is on a vertical micrometer screw, 
insulated by the hard rubber connector h. The distance apart of p, r and 
p' (to be denoted by d' and d") or any change of this distance (Y) are thus 
closely measurable in turns (mm.) of the screw. In a dry room this 
apparatus retains its charge Q very well and a great variety of fields are 
producible. 
3. Equations. — If we treat the case of the electrophorus as a closed cylin- 
drical field of cross section A, and if V 0 is the potential of the charged hard 
rubber surface, we may write 
A V 
where Q', V are the positive charge and potential in the top plate at a 
distance d' from the charged rubber surface at potential Vo and K' the 
specific inductive capacity of the dielectric medium. A similar equation 
holds if Q", V" are the charge and potential of the lower plate at a dis- 
tance d" from Vo with a layer of specific inductive capacity K" between. 
If the two plates are put in contact, V — V" 
If the two plates thus charged are then insulated and the top plate is 
