THERMAL EMISSIVITY OF THtN WIRES fN AIR. 
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at A, flowed through the water jacket and prevented the surface of the cylinder from 
becoming warmed by radiation and convection from the hot wire under experiment. 
Fig. 2 shows the water-jacketed cylinder in position. 
For inserting the wire to be tested into the enclosure-cylinder, it was attached at 
its two ends to a lamp-blacked brass carrier E, F (fig. 3), which could be slid, 
between guiding pins G, G (fig. 1), along the bottom of the inner cylinder into a 
definite position. The two ends of the wire were soldered to the tips a, a (fig. 3), of 
two wires a/3, a/3, 1 millim. in diameter, which were made of platinum-silver so as to 
Fig. 3. 
Fig. 4. 
have but a small heat conductivity. Two short pieces of platinum wire, 0’025 millim. 
in diameter, to be used as terminals for the voltmeter, were soldered with the slightest 
touch of solder to two points about 6 millims. distant from the ends a, a, of the wire 
under test, it having been determined that at this distance from the terminals a, a, 
the temperature would not be much lower than the average temperature of the 
wire. In the case of the two finest wires tested, having diameters of 0'025 and 
0'051 millim. respectively, it was not found necessary to adopt this arrangement, and 
the ends of the wires were themselves used as the voltmeter terminals, as seen in 
fig. 4; the platinum-silver wires, a/3, a/3, were, however, filed down quite thin to 
prevent the fine wire being cooled by heat being conducted away from its ends. The 
shaded portion of figs. 3 and 4 represents an ebonite cap used to insulate one end of 
the wire a, a, from the brass carrier E, F. 
To prevent draughts entering the enclosure-cylinder, each of its ends was closed 
with an ebonite plug, P, P (fig. 1) through two holes in which passed the platinum- 
silver wires, a/3, a/3, used as the terminals for the main current, and the fine platinum 
wire used for measuring the P. D. Connection was made between these latter and 
the wires which led to the voltmeter itself by their dipping into two mercury cups 
M, M (fig. 2), carried from the stand of the apparatus, and which were turned into 
position close to the holes in the ebonite plugs, after the latter had been inserted into 
position. 
For the experiments on the variation of resistance of the wires with temperature 
which were conducted during May, June, and July, 1888, the two ends of each wire 
were fastened to two thick rectangular copper bars, about 7 millims. thick by 20 
millims. wide. The fastening was effected by making a fine saw cut about 6 millims. 
