( 498 ) 
Fig. 3 represents the resistance mounted in the bath for the deter- 
mination of the zero. (comp. § 4). 
§ 3. Measurements with the WHEATSTONE’s bridge. 
The resistance was measured by means of a WHEATSTONE’s bridge, 
of which two branches were formed by coils of manganin wire. For 
this purpose pairs of about 7, 14 and 21 Ohms are wound, which 
may be introduced separately or together. The ratio of these resistances 
has been several times determined at different periods. Values 
were found for it, which differed by less than i/ioo °/o. 
The third branch was a resistance box of HARTMANN and BRAUN, 
also of manganin wire. This has been tested at the Reichsanstalt, 
where 0.00001 was given as the co-efficient of temperature, so that 
no correction for the temperature had to be applied. A resistance 
box of SIEMENS and HALsKE was arranged in parellel in order to 
apply variations of resistance smaller than 0.1 Ohm. 
In determining the dependance of the resistance on the tempe- 
rature the absolute values of the resistances are of less moment 
than their ratios. The latter have been determined a few times 
for the resistance box in the ordinary way and corrections have 
been applied wherever necessary. 
In the measurements with the WHEATSTONE’s bridge the contact 
resistances, if the resistance to be measured is small, may become 
of great influence. Even when all possible care is taken to keep 
the plugs clean and to tighten them well, yet with ordinary stops 
the resistance is not certain to less than 0.0005 Ohm. Therefore it 
was important that the platinum wire even at —180° C. should 
still have a resistance of about 30 Ohms, as in comparison with 
this resistance the connecting resistances may be entirely neglected. 
The connecting resistances in other 
parts of the circuit which may vary 
considerably from day tot day, were 
always eliminated as indicated by CaL- 
LENDAR. I took 4 wires instead of 3 
in order to render the elimination more 
complete. ‘The connections required were 
made possible by a commutator, made of 
copper in order to avoid thermo-electric 
effects — they occur for instance to a 
high degree with contacts in mercury 
cups. The annexed fig. 2 represents 
diagrammatically the commutator with 
its connections. 
