38 
MI-:SSKS. Ci. F. C. SEAKLE AND T. G. BEDFORD 
Let the couple experienced by the suspended coil, when the currents in the tixed 
and suspended coils are 0 and c respectively, be qCc dyne-centims. Then since 
H = 47rNC', when the magnetic force due to the secondary current is negligible, 
we have for the couple at any instant 
Couple = 
ri _ A/t 
</Lc = 7 ,“Vt , LL 
^ 47rXS dt 
+ 'Z 
dt 
If the time of vibration of the moving coil be so great compared with the time 
occupied by the cycle or semi-cycle, that the cycle or semi-cycle is completed before 
the coil has sensibly moved from its equilibrium j^osition, the angular momentum 
acquired by the coil is 
where K is the moment of inertia of the coil, and w is the angular velocity imparted 
to the coil by the electro-magnetic impulse. Now when C goes through either 
a cycle or a semi-cycle vanishes, and thus 
= .(4). 
Let 6 be the greatest angular displacement or “throw” of the coil, and ^ the 
restoring couple exerted by the suspension per radian of displacement. Then by the 
principle of the conservation of energy, we may equate the initial kinetic energy 
to the potential energy at end of swing and thus obtain -gKoj' = hf 
or K*(y = /“d.(5). 
The three constants q, K, and f are eliminated, and the “ constant ” of the 
dynamometer is determined in the following manner. Let a constant current C' 
flow in the primary circuit through the fixed coil, and while this current is passing, 
let the earth inductor be ini’erted so as to produce a known change, P, in the 
number of linkages of lines of induction with the secondary circuit. The time- 
integral of the current thereby produced is P/f8, and hence the initial angular 
momentum of the suspended coil is 
Kco' = r/C'P/S.(6). 
If (f) be the throw produced, we have by (5) co/co' = 0/<f), and hence by (G) and (3), 
r f'l’ 
\ccdt=--e .( 7 ). 
W = . 
