CONDUCTIVITIES OF IRON AND COPPER. 
571 
The arrangement of apparatus finally employed was that shown in fig. 1. One end 
of the experimental bar was immersed in a vessel of melted lead maintained at a 
constant temperature, and the rest of its length was exposed to the air, but 
protected from draughts and external radiation by a wide trough of sheet zinc, T T T, 
arranged as shown in the figure. The molten lead was maintained at a constant 
temperature by a thermostat, Th., which consisted of an air thermometer having its 
bulb immersed in the lead, and controlling, by means of a column of mercury in the 
U'shaped bend of its stem, the passage of the gas at m, the junction of the entrance 
and exit tubes a and h. By means of this arrangement the lead could be maintained 
at a temperature constant within a range of about half a degree for any length of 
time. 
f 
Fio-. 1. 
O 
A. Wooden screen. 
B. Wooden box lined with sheet ii on. 
C C. Metal bar. 
1>. Gas brerner. 
E. Pan containing melted lead. - 
T T T. Trough (of sheet zinc) protecting bai'. 
Tk. Thermostat (air thermometer with rner 
cury cut-off at di). 
c c. Thin copjier tube. 
i'j w. Cotton wool. 
*■ Strings supporting bar. 
IF IF. Blocks of wood supporting T. 
The bar was protected from the radiation from the heating apparatus by enclosing 
this apparatus in a box B, lined with sheet iron, and by screening oti* this box by a 
Avooden screen A, and lightly packing the space between the box and screen with 
cotton wool. The tube, c.c., shown in the figure was intended to sliield the part of 
the bar within the box fi’om irregular beating by the flame of the burner, and served 
also to prevent contact betw’een the cotton wool and hot bar. The space between 
this tube and the bar Avas packed at one end with asbestos to prevent a current of 
liot air passing out of the box along the bar. 
The thermo-electric couple employed was one of a good soft iron Avire and German 
siBer. The AAures were al^out ‘5 millim. in diameter, and the junctions Avere made by 
fusing the wires together under borax and filing down to the recjuired size. Repeated 
calibration shoAved that Avith tliis couple the deflection on the galvanometer scale was, 
4 D 2 
