16 TEMPERATURE FLUCTUATIONS IN THE HUMAN BODY. 



ance is kept constant regardless of the position of the contacts by the device 

 of two identical circuits, one of which is included between the sliding contacts 

 while the other is not; these circuits being so arranged mechanically that as 

 the resistance of one is decreased the resistance of the other is increased by 

 the same amount. This arrangement has the disadvantage that contact 

 resistance in the potentiometer circuit may affect not only the precision but 

 also the accuracy of the measurement. In choosing apparatus for similar 

 work, it should be noted that this is an unnecessary handicap, since the poten- 

 tiometer may be so designed as not to require any contact in this very import- 

 ant part of the circuit. Also for thermo-electrical work, the potentiometer 

 should have low resistance, so as to retain the full sensitiveness of the 

 apparatus. 



The potentiometer shunt is embedded in a block of paraffin, together with 

 the brass binding posts of the potentiometer; also a similar block is used where 

 the copper wires join the binding posts which are connected to the moving 

 contacts. None of the knife switches G 2 , T, 1, 2, 3, 4, etc., contain any 

 junction of dissimilar metals, the circuit being copper throughout. Moreover, 

 these switches are mounted at a distance of about 1 meter from the observer 

 and are operated by means of long wooden rods. This construction is adopted 

 in view of the fact that in some parts of the circuit, namely, in the poten- 

 tiometer, the shunt, the switches just mentioned, the galvanometer, and the 

 thermal-junction systems, it is very important to reduce as much as possible 

 all stray electromotive forces. 



The unavoidable junctions occur mostly in the form of reversed pairs; the 

 paraffin blocks, by surrounding both these junctions with a medium having an 

 appreciable mass, lessen the effect of transient fluctuations in the room temper- 

 ature; furthermore, as these blocks have a greater heat-carrying capacity than 

 air, they provide a better path for the transfer of heat between the two junc- 

 tions, thus tending to equalize the temperature of the two. The slender 

 wooden rods are used in connection with the switches as a further safeguard 

 at these points to prevent heating from the hand. 



Standard cell. The resistance M is furnished in the usual way with both 

 current- and potential-terminals. The standard cell N is a Weston Standard 

 Cell, No. 1565, provided with the usual certificate stating its electromotive 

 force. The following caution is expressed in the certificate: "To preserve 

 the constancy of this cell,itshould not be exposed to temperatures below 4 C, 

 and no current greater than 0.0001 ampere should be passed through it." 

 The insertion of the resistance of 10,000 ohms assures the fulfilment of this 

 latter requirement, and at the same time leaves an arrangement amply sensi- 

 tive. The switch Gi is in the form of a double-contact key, which remains 

 open unless pressed, thus preventing the standard cell circuit from remaining 

 closed accidentally. 



Galvanometer. The galvanometer is a reflecting instrument of the Deprez- 

 d'Arsonval type, manufactured by Siemens & Halske. Among the advantages 

 of this instrument might be mentioned first, that provision is made whereby 



