METHODS AND APPARATUS. 



31 



sends a current through the circuit B-P-D-R-A-B, this current being meas- 

 ured by the ammeter A. The contact P may be moved until the fall in 

 potential along P-D, due to the battery current, is just equal to the voltage 

 of the thermal-junction system TT. When this condition is fulfilled as 

 indicated by the absence of a galvanometer deflection the voltage of the 

 thermal-junction system may be computed directly from the constants of 

 the apparatus. As before, however, the temperature difference between the 

 junctions rather than their voltage is desired; and this may be obtained in 

 the usual way by calibration. 



The complete wiring diagram is shown in fig. 7. It will be noticed that, as 

 in the older type of apparatus, the ammeter A is replaced 

 by a standard cell N and galvanometer; also a variable 

 resistance V is included in the main circuit for adjusting 

 the current always to approximately the same value j 1 and 

 the galvanometer is provided with a number of switches 

 by means of which it may be connected to any one of a 

 series of thermal-junction pairs, or to the standard cell 

 circuit. The circuit has a great many features in common 

 with that of the earlier apparatus previously described in 

 detail, 2 and these need not be considered again at this 

 point. Some differences, however, will be noted. The 

 galvanometer sensitiveness is independent of the position 

 of the contact P, since moving this contact in no way 

 affects the resistance of the galvanometer circuit. The 

 sensitiveness being constant, a resistance W has been 

 inserted in the galvanometer circuit, and by this means 

 the sensitiveness adjusted until the deflection reads 

 directly in hundredths of a degree. No special provision 

 is made for reversing B and N, as experience with the 

 earlier apparatus showed this to be unnecessary. The 

 new apparatus differs from the old, also, in that the ther- 

 mometer must be detachable. Two heavy wires have 

 therefore been run to two metal blocks in each calorimeter, 

 and to these the thermometer can easily be attached. It will be noted in fig. 7 

 that each calorimeter is not provided with an entirely independent set of 

 connections, but that the chair calorimeter and calorimeter No. 4 are connected 

 in parallel, as are also calorimeters Nos. 3 and 5. This is allowable because 

 other considerations prevent the calorimeters thus joined from ever being 

 used at the same time. 



Fig. 6. Elementary wir- 

 ing diagram of mod- 

 ified apparatus. Cur- 

 rent from the battery 

 B flows through I'D, 

 this being part of the 

 slide wire CD, then 

 through the resistance 

 R and finally returns 

 to the battery through 

 the ammeter A. The 

 thermal junction sys- 

 tem TT is connected 

 through the galvan- 

 ometer G to the points 

 C and D, this circuit 

 being free from sliding 

 contact. 



'It will be seen that the method as described is approximate only, and not exact, since the 

 measuring current changes with each position of P. The error thus introduced may be made 

 negligibly small for any case by making R sufficiently large as compared with CD. The 

 resistance CD should be kept small from another standpoint as well, namely, that of sensi- 

 tiveness. By the use of some kind of compensating resistance, equal to CD and arranged 

 so as to be decreased as CD is increased, and vice-versa, the arrangement can be made exact. 



2 See pp. 14-22. 



