( 499 ) 
By means of the blocks 1 and 2 the resistance to be determined 
is included in one branch of the WHrArTsSTONE’s bridge, 3 and 4 are 
connected to one pair of leads ts and t4, 5 and 6 to the other ¢; and 4. 
By a proper arrangement of the plugs we measure successively 
1st tj +t; + R., And ty +t, + R, 3d ty + ty, 4° ts + tg. 
It is clear how the value of R will follow from this. 
No uncertainty remains here but that caused by the differences 
in the connecting resistances of the plugs in the commutator and 
of the resistance box used in the measurement. | 
The galvanometer was a thick wire THomson with a resistance 
of about 1.2 Ohm, and a sensitiveness of 1 m.m. at a distance of 
3 M. for 2.10 —8 Ampère. Period of oscillation about 10”. The 
galvanometer was suspended after the manner of JULIUS in a stand 
on a fixed pillar. So the zero was very quiet. Thermo-currents 
were so small that they could be neglected. They have been pre- 
vented as much as possible by making contacts of copper on copper 
and by bringing the soldering places of the platinum wire as close 
as possible together. In the branch of the galvanometer was a 
commutator with mereury contacts as described in Communication 
NPD. 
§ 4. The determination of the zero. 
The determination of the resistance at 0° C. offered at first many 
difficulties. 
It is absolutely necessary to enclose the wire in a glass vessel 
containing a liquid, as otherwise we have no certainty that it really 
becomes 0° C., when this vessel is placed in ice. Paraffin oil or 
petroleum as mentioned in $ 10 of Communication N®. 27, proved un- 
suitable, because it was difficult to clean the wire again. It is a 
good expedient to use a liquid which boils at a temperature a little 
above zero, and which by moderate heating under diminished pres- 
sure can be removed altogether. As such isopentane and amylene 
were used. 
In order that this liquid should leave nothing behind on the wire 
after evaporation, it is necessary to transfer it by distillation, the 
air being excluded, into a vessel in which the resistance is to be 
brought to 0° C. 
When we used thin conducting wires the arrangement of this 
vessel differed a little from that used with thick and hence stiff wires. 
Fig. 3 represents the apparatus as used with thin wires. The tube 
A, after being narrowed at the top and provided with a side tube 
d and ground joint, was carefully cleaned. Then the resistance was 
put in at the wide end, pushed up as far as e and the tube sealed 
