32 M. W. Siemens on a new reproducible Standard y 
the tube and ran through it into the other. By the time the 
mercury in the two vessels was at the same level, the tube was 
generally found free from air-bubbles. Thick amalgamated 
copper wires were now inserted into the mercury through the 
top of the receivers, and the resistance of the tube was compared 
with that of one of Jacobi’s standards by means of the bridge 
above described**. 
The resistance of the conducting rods was determined by 
plunging the amalgamated copper cylinder into a vessel filled 
with mercury. It was found to be quite evanescent in compa- 
rison with the resistance of the tubes. 
The experiments whose results are collected in the following 
Table were conducted as follows:—In the first position of the 
commutator, the slide BB was moved to such a position that, 
on lowering the contact lever I, the galvanometer exhibited no 
permanent deviation. The resistances to be compared were then 
changed by means of the commutator, and the slide was again 
adjusted. These two positions of the slide are given in the 
columns headed a and 6. If the observations were free from 
error, the sum of these magnitudes would be exactly 1000, 
which was gencrally the case, or very nearly so. We must here 
remark that, after establishing the equality of the currents, a 
small temporary deviation of several divisions, indicating a greater 
resistance on the part of the closely packed coils of Jacobi’s 
standard, was observed immediately on completing the circuit. 
As on breaking the circuit an opposite deviation of the same 
magnitude resulted, this phenomenon was obviously to be attri- 
buted to the extra current of the wire coil of the Jacobi’s stand- 
ard. It appeared, moreover, that on a long continuance of the 
current, the mercury in the tube began to grow warm, even 
though only a single cell of a Daniell’s battery was employed. 
On account of the slow oscillation of my mirror and the resist- 
ance encountered by its prolongations, the error arising from this 
cause was easily eliminated by allowing only short currents to 
traverse the instrument. The slide was always so placed that, 
immediately on completing the circuit, there was a shght move- 
ment towards the left, which, on the mercury becoming warm 
owing to the continuance of the current, gradually passed over 
* At first, instead of amalgamated copper wires, iron cylinders were used 
as conductors ; under these circumstances, however, we found that there was 
a very considerable resistance to the passage of the current from the iron 
to the mercury, even though the surface of the former was perfectly clean. 
This resistance, which was ralso considerable when unamalgamated copper 
wire was employed, was particularly strong when the cylinder had been 
some time exposed to the atmosphere after “having been cleaned, so that it 
is probable that this phenomenon is to be attributed to the gaseous con- 
densation on the surface of the metal. 
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