244 
4 
per microvolt in scale divisions at a distance of 1000 divisions we 
find the general formula 
Pis 0,57 Te Kla ra 
and in the two cases which have been calculated: 
= 8. resp, ADEN 
I have actually attained sensitivities of this order, also the three- 
fold sensitivity in the case of 10 2 total resistance. By providing 
the galvanometer with a magnetic shunt, which can be moved by 
a screw, | was able to make H continuously variable, between the 
greatest value mentioned and '/, of it. By the aid of this shunt we 
are able to use one single instrument for resistances from 11 to 
100 2 eg. and to bring the instrument at once in the aperiodic 
condition for any resistance within these limits. For the weaker 
magnetic fields got in this way m will be reduced to 2 or 3. All 
the more reason not to take A/A, greater than 3. 
Remains a very important matter viz. how it is possible to realize 
the required very small restoring torque. For a strip with rectan- 
gular section of sides a and 6 and length /, which is twisted, 
we have 
ae 
31 
when 6 is small with respect to a. G is the torsion modulus. 
One might try to cut small strips of metal foil in order to get 
b very small. The ordinary silver foil for instance has a thickness 
of 0,2 u. Experiments showed that these strips cause a many times 
greater torque than that calculated from their dimensions. Apparently 
the beaten metal departs too much from the simple shape supposed 
in calculating the formula, its thickness being very uneven and its 
surface very rugged. 
Therefore 1 have made silver foil of 0.4 to 0.7 u thickness by 
electrolysis, for instance by precipitating the metal on zine and 
afterwards dissolving the zinc in a weak acid. By means of a razor 
attached to a dividing engine, one is able to cut narrow strips of 
this foil, if necessary even of 0,02 mm. width. 
For a silver strip 0.5 u thick, 0.1 mm. wide and 10 mm. long the 
formula gives D—0.00012. Two of these strips would thus give 
a torque of only one tenth of the smallest one required. Repeated 
experiments showed that by very careful treatment one gets with 
such strips a torque which is 2 or 3 times the calculated one. From 
the dimensions mentioned one finds further for the resistance 3 2 
