14 TEMPERATURE FLUCTUATIONS IN THE HUMAN BODY. 



that the combination gives a resistance of about 10 ohms, variable in tenths 

 of ohms. Vi and V 2 are provided should even more adjustment be necessary, 

 and have a resistance of 5 ohms each. This gives 20 ohms for the sum of the 

 variable resistances, about half of which should be in circuit under normal 

 conditions, so as to allow adjustment in either direction. The value for R 

 is accordingly 90 (20^2) =80 ohms. 



To determine whether the galvanometer is sufficiently sensitive, it will be 

 simplest to consider the case of a measurement being made in which the circuit 

 is not quite balanced, but owing to the insensitiveness of the galvanometer it 

 appears to be so. The largest current that can pass through the galvanometer 

 and still be undetected is 15 X 10~ 10 amperes. This current in flowing through 

 a resistance of 64 ohms, composed of the galvanometer and thermal -junction 

 system, causes a fall in potential of 15 X 10 -10 X 64 or about 10 -7 volts. 

 Since 0.01 C. corresponds to 0.4 X 10 -6 volts, or 4 X 10~ 7 volts, it is seen 

 that the error due to insensitiveness of the galvanometer corresponds to about 

 0.0025 C. and is therefore entirely negligible. 



CONSTRUCTION OF THE APPARATUS. 



Having determined the constants, or electrical dimensions, of the various 

 pieces of apparatus considered in fig. 2, the actual method of construction and 

 installation will be given in some detail. This seems advisable, since in some 

 cases the peculiar experimental conditions to be met required a special form 

 of construction; while in other cases some of the more elaborate pieces of 

 apparatus available, which were not ideally adapted to the work, have been 

 made to conform more closely to the desired qualifications. 



MEASURING INSTRUMENTS. 



Storage cell. The storage cell B is one of a battery of six cells, having a 

 normal rate of three-quarters of an ampere and provided with separate 

 terminals for each individual cell. A suitable resistance, not shown in fig. 2, 

 is arranged so that it can be connected across the battery when desired. The 

 separate terminals permit each cell to be used singly, and the resistance is 

 used partially to discharge the battery when freshly charged, thus causing its 

 voltage to be more nearly constant. 



Both the storage cell and the standard cell are provided with reversing 

 switches, which are mounted on a single base and arranged so as to be operated 

 together by means of one handle. This is done for the following reason: The 

 electromotive force of the thermal-junction system, standard cell, and storage 

 cell must all act so as to have a tendency to send current through the main 

 circuit in the same direction. For the purpose of keeping the voltage set up 

 by the junction unidirectional, the temperature of the oven is always kept 

 higher than that of the body. In order to provide for a possible decrease in the 

 oven temperature, with a consequent reversal of this voltage, the storage cell 

 and standard cell are furnished with the reversing switches just mentioned. 



The resistances R, Vi, V 2 , V 3 , V 4 , S, M, and O, as well as the coils in the 

 potentiometer, are of manganin. Vi and V 2 are each arranged to be short- 



