THE MECHANICAL EQUIVALENT OF HEAT. 
397 
Method of finding B. 
and P 3 (see fig. 2 , p, 382) were blocks of paraffin. The wires leading from the 
calorimeter to the junctions on the ebonite rod H passed through the block P„, down 
which holes were then bored until the wires were exposed. These exposed portions 
were carefully amalgamated and the holes filled with mercury. Two thick pieces of 
copper were so bent that communication could be established between any of the holes 
in Pi and the two troughs in P 3 . When placed in position, the ends of these con¬ 
necting strips rested on the conductors at the bottom of the holes. Two thick wires 
led from the troughs in to the resistance box, and, in order to bring their resistance 
up to a convenient quantity, a coil was introduced into one of these leads. 
Denoting the total resistance of the conductors from the box to the lid of the 
calorimeter by ?q, r^, it is obvious that all the following resistances could be 
taken by a movement of the connectors between Pj and Pg, viz., rj + E, + rg, 
’2 + ’3 -f The determinations were always made in the order thus given. 
If N|, Ng, Ng, and are the resulting numbers, we have 
E = {(N3 -f Ng) - (N, + N,)]/2. 
The galvanometer used had a resistance of about Scj, and was fitted with a micro¬ 
scope containing a micrometer eyepiece by Zeiss. The image of a black line placed 
on a slip of paper in front of the mirror was thrown on to the micrometer scale. 
A galvanometer fitted in this way is a very convenient instrument, for the swings 
can, with practice, be read with greater ease and accuracy than when the ordinary 
lamp and screen arrangement are used. 
When a resistance was taken (whether of the coil or of a platinum thermometer) 
the order of the observations was as follows :— 
Having determined between which plugs the resistance lay, the swing corresponding 
to the lower one was taken, the battery inverted and the swing in the opposite 
direction read. This process was repeated when the higher plug was in place. In 
this way all thermal or other effects are practically eliminated, and the method is 
independent of small changes in the zero position of the spot. 
In some cases, as when working with platinum thermometers at high temperatures, 
the thermal effects are considerable, and the position of the zero point becomes so 
changed that the spot is thrown off the scale when connection with the galvanometer 
is made, although the battery circuit is not connected. To meet this difficulty a 
special key was constructed. 
The ends of a and h (fig. 5) and also of e anciy are connected with the galvanometer 
circuit, the ends of c and d with the battery. The screw at a is so arranged that, 
when the key is untouched, the galvanometer circuit from the box is completed. 
The position of the spot at any time, therefore, is that due to the influence of any 
