Physics. — “A moving coil galvanometer of high sensitivity”. By 
Prof. F. Zernike. (Communicated by Prof. H. Haga). 
(Communicated at the meeting of October 29, 1921). 
Introduction. The problem to determine the conditions for which 
a moving coil galvanometer reaches maximum sensitivity, has been 
frequently discussed in the literature of the subject *). 
The result attained is in short as follows: for any fixed period 
of oscillation the sensitivity varies inversely as the root of the 
moment of inertia K of the moving system. 
Now the torque D of the suspension cannot be decreased beyond 
the limit determined by the smallest dimensions of suspension strip 
available, which limit until recently was 0,2 C.G.S. Hence K, which 
is proportional to D, cannot be decreased indefinitely. 
In recent years several galvanometers have been constructed with 
much smaller torques. Even if there was no limit to the smallness 
of D, the sensitivity would still be restricted as A cannot be 
indefinitely decreased because of the presence of the galvanometer- 
mirror. Indeed it is very remarkable that in the above mentioned 
discussions this important detail of the instrument has hardly been 
taken into account. Only EintHoven ’) has laid stress upon the fact 
that by judging the sensitivity of galvanometers the size of the 
mirror ought to be taken into account. As is well known, his studies 
led to the construction of another type, the stringgalvanometer, 
hence do not answer the question which we put: how to make a 
reflecting galvanometer with moving coil as sensitive as possible. 
To solve this question I will start with a given mirror. Up till 
now the mirror was considered to be adetrimental though a necessary 
addition, because it increases the moment of inertia. Indeed I found 
the moment of inertia of the mirror to be from 1 to 3°/, of the 
whole system in different commercial instruments. I will invert this 
') See ia, W. JAEGER, Z f. Instrumentenk. 23, 261 en 533 (1903). 
W. P. Warre, Phys. Rev. 19, 305 (1904). 
W. J. H. Morr, these Proc. 
Discussed at length by W. Jarcer, Elektrische Messtechnik, Leipzig 
1917, pg. 204 sqq. 
2) W. EINTHOVEN, Ann. d. Physik 12, 1062 (1903). 
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