897 
to the first group the deflection in which obeys a linear law. The 
electrodynamometer is a prototype of the second class, in which a 
quadratic law prevails. 
df the movable part of an instrument of the first group be mecha- 
nically coupled to the movable part of an instrument of the second 
type, the rotating axes of the two instruments being made to coincide, 
and we send the same current through both instruments, taking 
care that the constituents of the movable part try to move it in an 
opposite direction, we-have got a new instrument type showing a 
few peculiarities which are not to be found in either of the con- 
stituent parts. 
If / be the current strength, the deflection y, the constant of the 
galvanometrie part a, the dynamometric constant 6, we can put 
agen ame ed bli 
if the constituent parts are separately considered. 
For the two parts used together we find: 
ye Ast ds 
(Ne en hale Vn Fiel (1) 
This is the equation for a parabola. Therefore we may call in- 
struments formed by the combination of an instrument with a linear 
law of deflection with one obeying a quadratic law, instruments with 
a parabolic law of deflection. 
From the equation 1) giving the deflection in terms of the current 
strength we immediately find, that there will be no deflection either 
with a current /==0 or when 
oe WA er ie Ae DEN 
If we eall positive the divection of the movement of the movable 
system when the dynamometric part has been short-circuited we 
find that a maximal positive deflection occurs when 
b? 
i ¥ 
2a 
the deflection being 
| L (3) 
¢ = — A 3 
4a° 
ans 57 
Any current between /— 0 and /—-— gives a positive deflection; 
a 
b? . . 
currents larger than /— — cause negative deflections. We also get 
qa 
a negative deflection with currents smaller than /=0 i. e. after 
commutation of the normal direction of the current. 
