METHODS AND APPARATUS. 



11 



THEORY OF METHOD OF MEASUREMENT USED. 



The thermal-junction method is based upon the well-known principle that 

 if a junction of two dissimilar metals be heated, a difference of potential 

 between the two will result; and if the circuit be closed through a galvanom- 

 eter, a current will be produced and the galvanometer deflected. In order 

 that small temperature fluctuations may be measured with precision, use is 

 made of a second junction in series with and opposed to the first, this second 

 junction being kept at a constant known temperature somewhere in the region 

 of the temperature to be measured. By this means the net electromotive force 

 is made very small, so that slight changes due to fluctuations of the unknown 

 temperature form a large percentage of the whole. The use of deflections in 

 this way, although convenient and simple, has one disadvantage, in that the 

 resistance of the galvanometer and conducting wires must be taken into 

 account, since a change in the circuit resistance would cause an inversely pro- 

 portional change in the deflection. With the appa- 

 ratus used in these body-temperature experiments, 

 a fluctuation of more than 1.2 C. in the galvan- 

 ometer temperature would not have been allowable. 

 Moreover, the effective voltage at the galvanometer 

 terminals, being the voltage of the thermal-junction 

 system minus the fall in potential along the wires, 

 would have been reduced to about 75 per cent of 

 its total value. 



These conditions, although not prohibitive, were 

 nevertheless regarded as undesirable, so that a 

 "null" method, by means of which they were en- 

 tirely -avoided, was finally decided upon. This 

 "null" or "zero" method is a modification of the 

 ordinary potentiometer method for the measure- 

 ment of electromotive forces, and consists in balanc- 

 ing the electromotive force of the thermal-junction 

 system against the fall in potential across a section 



of a standardized slide wire in which a known current is maintained. An 

 elementary diagram of connections is shown in fig. 1. The battery B sends 

 a current through the circuit B-D-C-A-B, the value of the current being 

 measured by the ammeter A. CD is a uniform slide wire of known resistance, 

 to one end of which, as at C, is attached one end of the thermal-junction sys- 

 tem TT. The other end of the thermal-junction system is connected through 

 the galvanometer G to a movable contact which may be touched at various 

 points along the slide wire until a point P is found at which no galvanometer 

 deflection results. The voltage of the thermal-junction system can then be 

 calculated, if desired, this being the product of the current expressed in 

 amperes and the resistance CP expressed in ohms. The voltage of the system 

 is, however, usually of secondary importance; what is desired is the tempera- 



Fig. 1. Elementary wiring dia- 

 gram of apparatus. Current 

 from the battery B flows 

 through the slide wire DC 

 and returns to the battery 

 through the ammeter A. 

 The thermal-junction system 

 TT, with the galvanometer 

 G, is connected at one end to 

 C, and at the other to a 

 movable contact which may 

 be touched at any point along 

 the slide wire. 



