( 807 ) 
correction necessary for the force exerted upon the second straight 
element of the balance (i. e. that outside the pole-gap). The sum 
total of these positive and negative corrections came to some units 
per thousand. 
The greatest care had to be devoted to the absolute value of i, 
which was measured by means of an accurate ammeter by Siemens 
and Halske. This was calibrated in absolute -amperes by comparing 
on the potentiometer the potential difference between the terminals 
of an international ohm (or for the stronger currents of 0,1 Si) with 
the electromotive force of a Weston cadmium cell. For the requisite 
accuracy of the measurements the influence of neighbouring instruments 
or currents upon the ammeter, or of its position in the earth’s field 
were by no means negligible; we got rid of almost all these irregularities 
by a suitable adjustment of the distances and of the positions of the 
rheostats, and we eliminated further possible remaining errors by so 
connecting all the conductors that the currents in all except the am¬ 
meters could be reversed at the same time. Finally we always used 
the ammeters in the same position with respect to the earth’s field 
as that in which they had been calibrated. 
When all calculations and corrections had been completed it was 
found that the strength of the standard field was 9857 gauss according 
to this method. 
The second method by which the value of the same field was 
found consisted of the sudden withdrawal from between the poles 
of the magnet of a coil of wire of which the area encircled 
by the current was known. The change thus caused in the number 
of induction lines embraced by the coil was compared by means of 
a ballistic galvanometer with the number of induction lines embraced 
by a solenoid the dimensions of which were accurately known. 
The coil consisted of 19 turns of silk-insulated wire, 0.25 mm. 
thick, wound round a cylinder of ebonite, 20 mm. in diameter. 
The dimensions were obtained by various measurements with the 
micrometer screw and the dividing engine, and were repeatedly 
controlled. At the same time a control coil was constructed by 
winding bare copper wire in a helical groove cut in the curved 
surface of a cylinder of ebonite; the area encircled by the current 
was then measured for this control coil by the same methods and 
with the same precautions as were adopted in the case of the first. 
The ratio of the two was in agreement with the ratio of the deflec¬ 
tions of the ballistic galvanometer which were obtained by connecting 
the two coils in series with the galvanometer and then withdrawing 
them successively from an unchanged magnetic field. We may further 
54 
Proceedings Royal Acad. Amsterdam. VoL XII. 
