THE MECHANICAL EQUIVALENT OF HEAT. 
383 
even in bright day-light, by either of the observers when in the situations they 
occupied during the experiments. The extreme sensitiveness of this galvanometer 
will be illustrated when dealing with the comparison of our Clark cells. 
The method of maintaining the ends of the coil at a constant potential difference 
has been indicated in Section 1. The experimental accuracy of the method depends 
entirely upon the action of the rheostat at K (fig. 1, p. 370). This instrument should 
be capable of a fine, as well as a rough, adjustment, and, above all, there should be no 
possibility of uncertain contact. 
The majority of rheostats establish connection by the pressure of a sliding piece 
upon a wire, which is, at the best, an unreliable form of contact. The instrument 
devised by us has, however, proved to be so free from this defect that we describe it 
at some length. 
It is well known that the electrical contact between a moving platinum wire and 
mercury is, under ordinary circumstances, an unsatisfactory one ; its uncertain nature 
being probably due to the air-film attached to the wire.* We found that in vacuo 
the contact was in every way satisfactory. 
A and B (see Plate 2, fig. 3) are two glass tubes about 7 feet long, with a platinum 
wire running through the interior of each—the ends of the wires projecting at A and B. 
The resistance of the wire in A is about 6 ohms per foot, and of that in 5 P7 ohm 
per foot. C and D are two wide tubes about 3^ feet tong, closed at the lower ends. 
A and B were carefully filled through a capillary tube with hot mercury, and 
inverted so as to stand in C and D. Practically, A and B are two barometers 
having tubes of about twice the usual length. C and D are firmly fixed in a stand, 
and A and B are raised or lowered by handles at the side, connected with gearing 
wheels which move two racks, fixed upon the rods carrying A and B. In their 
lowest position the barometer column reaches almost to the top of the tubes; in their 
highest, about 4 feet of the wire is left above the mercury. Spiral wires are attached 
to the projecting ends of the platinum wire at the top, and the ends of a copper fork, 
carrying a screw connector [E) at the bend, communicate with the mercury in 
Cand D. 
The current enters at jP, passes down the wires in A and B (which are thus in 
parallel arc), and leaves the instrument at E. The wire from F to B is so arranged 
that additional resistance can be placed in it if required. 
The mean resistance of the calorimeter coil used by us this year was about S'Sw; 
the minimum external resistance about l’3aj. The E.M.F. of each storage cell was 
about 1'4 times the E.M.F. of one Clark cell. Denoting our external resistance by r, 
and the coil resistance by P, it is evident that (when the number of storage and 
Clark cells were equal) an approximate balance would be obtained if (R + ^’)/R = 1‘4, 
which gives r = 3*4, i.e., wRen the rheostat resistance wms about 2‘lco. 
* If the wire is amalgamated the mercui’y is apt to cling to it when the wire is withdrawn. 
