DE. A. MATTHIESSEN AND M. VON BOSE ON THE INFLUENCE OF 
to which the normal wire (in the glass cylinder G) is soldered, are also 4-5 millims. 
thick. The reason why such thick wires were chosen was to make any difference in 
their resistance, caused by the change of temperature in the room or by the heating of 
the ends in the oil-bath, so small that no correction was necessary. This was proved to 
be the case by the following experiment : — After having soldered a wire in the trough 
to the ends of the thick copper wires, and determined its resistance with the normal 
Avire generally used, the wire F at e Avas heated with the 6-Bunsen burner much above 
100° C., and the resistance of the circuit was again determined whilst the Avire was at 
that temperature, when it was found to have increased only 0*08 per cent. ; we did not, 
therefore, consider it necessary to make any correction for the increase of resistance 
caused by the heating of the ends of the thick wires in the trough. The resistance of 
the copper wires was determined at the ordinary temperature, and brought into calcu- 
lation without further correction. Before the commencement of each series, all the 
ends of the Avires dipping into the mercury- cups were carefully re-amalgamated. L, L' 
are the two commutators fitting into four mercury-cups at o. 
The Avii’e stretched on the board H is of german silver instead of copper, as was for- 
merly described; its half-length was 4550 millims. The length of the board is about 
1500 millims.; the wire, therefore, was wound backwards and forwards several times on 
the one side ; this is not visible in the figure. By using normal Avires of different resist- 
ances, and by choosing proper lengths of the wire to be tested, it was always possible to 
begin the obserA^ations Avith the block M Avithin 100 millims. of the middle of the wire. 
Great care was taken to lift the block M off the wire when it was moved, in order to 
prevent as much as possible its wearing. It may be mentioned that, although we 
