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898 
For very smail currents we have a sensibility = «, which means 
that a current /—a gives a deflection of one division of the scale. 
Near the second position of equilibrium a current inerement of — a 
or a current decrement of + « augments a positive or decreases a 
negative deflection by 1 part of the scale. The absolute sensibility 
of the instrument is the same either in the first or in the second 
zero position. 
_ The relative sensibility i.e. the ratio of the current increment 
causing an increase of one part of the scale into the current strength 
approaches to 0 in the first zero position ; in the second zero position 
2 
it is 5 which will generally be a very large figure. Instead of in 
this way we preferably calculate the relative sensibility for the cur- 
rent causing the largest deflection at which the linear law still holds 
good. Expressed in this way we can say that the relative sensibility 
will generally be of the order of 200—500. But in the second zero 
position we can easily obtain a relative sensibility of 100000 or 
more. Even with a moderate ratio between a and 4? we obtain 
very high figures for the sensibility near the second zero position. 
As a matter of course we can only use very small parts of the 
parabola in most cases. We need scarcely point to the fact that for- 
mula (1) is only available for deflections for which both the linear 
law for the galvanometer and the square law for the dynamometer 
hold good. 
In instruments of this kind the thing we aim at is a high relative 
sensibility near the second zero position. As an example we might 
take an instrument giving a full scale deflection at 10 milliamperes, 
and which has its second zero position at say 1 ampere; in this 
case a change in the current strength of only one percent would 
cause the spot to leave the scale. 
Many years ago I had an instrument of this kind made for me 
in a rather simple form. I used it as an indicator that the current 
in a potentiometer remained constant during the measurements so 
as not to be obliged to correct the setting of the potentiometer by 
means of the Weston-element. For this purpose the parabolic indi- 
cator proved to be very useful. We can easily give such an instru- 
ment any relative or absolute sensibility. With a shunt to the galvano- 
metric part we diminish the relative sensibility ; with a shunt to the 
dynamometrical part we can increase it. A shunt to the complete 
instrument increases the current strength for the second zero-position. 
One might expect a parabolic instrument to make an excellent 
standard- or normal instrument. But there are a few objections to 
