MR. 0. V. BOYS OR THE RADIO-MICROMETER. 
171 
equal to just as, when the conditions with respect to weight were considered,^ was 
taken as equal to 1. 
As in the rest of this paper the best conditions with respect to weight will no more 
be considered, but only those relating to moment of inertia, it will be well here to 
specify the meaning of the several symbols that will be used. The arithmetical values 
of the fixed cjuantities are, as before, included within parentheses. 
(9-49 X 10-3) K. 
k 
(6-742 X 106) 0. 
r. 
I 
n. 
(2-2375 X 10-3) 
(1-642 X 106) V. 
a. 
The moment of inertia of pJate, mirror, and stem, the dead 
moment of inertia. 
The moment of inertia of the copper hoop, the variable 
moment of inertia. 
The resistance of the plate, the dead resistance. 
The resistance of the copper hoop, the variable resistance. 
The length of the rectangle. 
The number of turns of wire. 
The moment of inertia of the unit piece of copper (1 X ’1 X '01) 
at 5 mm. from the axis. 
The resistance of the unit piece of copper. 
The sectional area of the wire (-1 X "01 being considered unity). 
Since the formulae already given are now equally true with respect to moment of 
inertia, it will be unnecessary to do more than barely state them here for future reference. 
For best sectional area (n turns) a = 
. . . (7), 
for best length (a turns) Ird = (2a — 1) p -f ■ • ■ 
■ . . (8), 
for best a (one turn) r = C -f- 2p /sj . . . 
■■ . ■ (y), 
for best k (one turn) Z: = K (^1 -f 2j:> ' ' ' 
. . . (10). 
for best E^- (one turn) E/ = ---- 
^ ^ 8 (v/tCKar)+pn-} 
. . . (11). 
P = 4 + any excess allowed for soldering. 
Ihe following arithmetical results are obtained from some of these expressions, 
making p = ^ :— 
