240 
—_ 
and state: the coil is a detrimental though a necessary addition to 
the mirror, and one should take care that the total moment of inertia 
does not become much greater than that of the mirror. 
The following formulae will show clearly what may be attained 
in this respect. Afterwards | shall prove that for the technical 
construction according to these principles one can calculate every 
detail of construction about in the same way as an engineer cal- 
culates a dynamo, at the same time [ shall give the data of actually 
constructed galvanometers, as they are put on the market by Kipp 
and Sons Ltd., Delft. 
Calculation. For the voltage sensitivity we have 
Hf 
me (1) 
as condition for the limit of aperiodicity 
TDW. eN 
2r 
and for undamped oseillations 
Dele ik 
7: 
Here and further on the letters have the following meaning: 
P rotation in consequence of unit e.m.f. in the circuit, 
H intensity of the magnetic field, 
f winding surface of the coil, 
D torque for unit angular displacement, 
K moment of inertia of the whole system, 
KK, moment of inertia of the coil, 
r resistance of the whole circuit, 
r, resistance of the coil, 
T’ complete period of the undamped oscillations, 
me == A nin. : 
in which all quantities are to be expressed in electromagnetic C.G.S. 
units. 
Eliminating Hf and D from (4), (2) and (3) we get: 
ne 
ez 
re) a 
for the voltage sensitivity. Now it is well known that the current 
sensitivity of any galvanometer is proportional to V7, the voltage 
sensitivity inversely proportional to this. Therefore the power 
sensitivity (Watt-sensitivity) is independent of the resistance. It is 
apparent from (4) that we can only increase this power sensitivity 
