THE CALORIMETER SYSTEM AND MEASUREMENT OP HEAT. 147 



orimeter room is always of such uniformity as to cause only slight vari- 

 ations in the electro-motive force of the batteries, small alterations in the 

 variable resistance serve to keep the standard current well in hand. 

 Variations from hour to hour rarely amount to more than 2 or 3 mm. 

 on a deflection of 1 20 mm. This method of obtaining a constant current 

 is, for the purpose of this research, sufficiently accurate and on the 

 whole distinctly preferable to the use of a standard cell. 



The adaptation of the mercury contact to this form of bridge is shown 

 in figure 43. By depressing the key marked S the copper links enter 

 the mercury in the cups in such a manner that the bridge and battery 

 circuits are made before the galvanometer circuit. In this key it is 

 necessary to have five mercury cups instead of four, as in the keys 

 connecting the thermal junction systems. The fifth mercury cup is 

 made outside of the square inclosing the regular cups, and is seen at 

 the right, immediately in line with the standard resistance coil in fig- 

 ure 40. The fact that these bridge systems were added to the mercury 

 contact switch after it was first built explains the irregularity of the 

 position of the extra mercury cups, not only in this standard circuit, 

 but also in the four bridge circuits controlled by the four keys on the 

 bottom row. 



The flow of the current through the different connections, when the 

 key is depressed, may be followed very readily if it is borne in mind that 

 all the copper links connect cups with those that are immediately above. 

 On the whole switch there is but one exception to this rule, and that 

 is the connection for the battery circuit for the standard system. Here 

 the copper link connects the extra mercury cup with the nearest cup. 



In figure 40 the top of the switch at the right is removed and shown 

 in such a position that were it brought over like the cover of a book 

 the copper keys would fall over the proper mercury cups. Hence it 

 can be seen that the two upper rows are for the cups connecting with 

 the two thermal junction systems, while the first key on the next to the 

 lower row corresponds to the standard bridge system. Indeed, the 

 copper link extending on one side is clearly seen. This link dips into 

 the extra mercury cup. The four lower keys, each with one or more 

 extension links, belong to the four other bridge systems. 



Of these four systems, one controls the measurement of temperature 

 changes in the copper thermometer suspended in the air in the calo- 

 rimeter chamber. This is designated as system No. 5, and is controlled 

 by the lower left-hand key (fig. 43). The standard coil of zero tem- 

 perature coefficient wire used for comparison with the copper ther- 

 mometer No. 5, i. e. , the coil corresponding to D in the bridge system 

 (fig. 44), is wound on a small hard-rubber spool, and is provided with 



