THE CALORIMETER SYSTEM, AND MEASUREMENT OF HEAT. 149 



The key marked W is not as yet in practical use. Experiments are 

 in progress to utilize this method of temperature measurement to obtain 

 the differences in temperature of the ingoing and outcoming water cur- 

 rents, temperature differences now measured by mercurial thermom- 

 eters. (See p. 133.) The coil W is immediately at the right of the 

 keys marked S and No. 7. 



With the large area of wire exposed in the copper thermometer coils 

 No. 5 and No. 7, and the consequent rapid radiation, we have found 

 that the slight amount of current, 0.03 ampere, produces no appre- 

 ciable local heating effect in the coils ; hence the reading of the gal- 

 vanometer may be taken with the circuit closed and after the deflec- 

 tion has become constant. The method of reading the deflection ob- 

 tained when the R coil is used is that of observing the amplitude of 

 the first swing. As the coil for the R thermometer is compactly wound, 

 and therefore does not radiate heat readily, the passage of the current 

 through it heats the coil, giving rise to erroneous readings. The 

 amplitude of the first swing has been found to be sufficiently accurate 

 for these readings, and the rectal thermometers are calibrated to be read 

 under these conditions. 



At the upper left-hand side of figure 43 the various coils and their 

 connections with the switch are shown. It is thus seen that coils No. 

 5, No. 7, and R all have a common return wire from the calorimeter 

 chamber. 



The connections for the coils for the bridge system inside the calo- 

 rimeter i. e., copper thermometer for the air (No. 5), thermometer 

 for the copper walls (No. 7), and the rectal thermometer are con- 

 ducted from the mercury switch through a cable having a number of 

 strands of heavy flexible wires to a plug switch fastened to the copper 

 wall of the calorimeter. This cable is seen in figures 29 and 30, while 

 the location of the plug switch is seen at M, figure 33. The connections 

 for the bridge systems corresponding to No. 5, No. 7, and R are made 

 by inserting tapered plugs into the different sections of this switch. 

 Other sections (there are ten in all) can be used for other electric cir- 

 cuits, such as the telephone, wires for electrical check experiments, 

 and connections for the bicycle ergoineter (see p. 164), but they are 

 connected with much smaller (No. 18) wires with a set of switches 

 under the edge of the observer's table. (See fig. 37.) 



In the connections between the thermal junction system and the gal- 

 vanometer, relatively large changes in resistance in the contacts are 

 without effect on the deflection of the galvanometer ; but it is readily 

 seen that with the bridge systems the matter is very different. Here 

 the mercury contacts become a part of the connection between G' and 



