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under these circumstances action and reaction are acting within one 
and the same solid body, the source of error 2 and eventually 3 
are wholly eliminated. We may therefore concentrate our attention 
on the actions of the magnetic field of the earth. 
§ 2. Description of the apparatus. 
Fig. 1 shows the apparatus schematically. The proportions have 
not been indicated rightly. S is the soft-iron cylinder. Length 23 em, 
thickness 3 mm. Over a length of 21.5 em it is wound with ena- 
melled copper wire of the A. E. G. 0.08 mm thick. The whole 
resistance of this wire and the two wires p, q through which the current 
is led, is 320 2. p and q are silver hair wires of HARTMANN and 
Braun, thickness 0.015 mm. Preliminary experiments proved that 
they did not influence the torsion of the suspending wire 7. The 
wire r is fixed to a turning disk /, which makes it possible to 
turn the eylinder about its vertical axis through an angle of 360°. 
Beside the cylinder the figure shows a pendulum, consisting in an 
ivory ball 7 ckarged with lead and suspended by a steel wire dr ; 
length a little below 1 m. and thickness '/, mm. Point of suspension g. 
Bigs i: 
If / oscillates, the steel wire alternately makes contact with a 
and 6, two copper wires represented in transverse section and 
placed about 12 cm below g. The distance between a and b is 
about '/, mm. Further cd and de are two resistances of 67.5 2 each, 
through which the current of a battery of accumulators flows from 
«to e. The further connections are evident from the figure. 
83 
Proceedings Royal Acad. Amsterdam. Vol. XVIII. 
