for Thermocouple Measurements. 



fiG9 



sent through 100" or 100 + 900 or 100 + 900 + 9000, in order 

 to get three grades of sensitiveness : it then passes through 20 

 coils of 0*l w each, a slide-wire of a little more than 0'1 ohm. 



Fig. 1. 



Hh 



-^VWVVUvvvvvvvvvrvHvvv 

 t 



9000 900 100 



l^ I8f 02 01 



and through the adjustable resistance R back to the accumu- 

 lator. The fixed resistance of 100, 1000, or 10,000 ohms is 

 shunted by a cadmium cell C and galvanometer Q, and R 

 adjusted till the galvanometer is balanced. The thermocouple 

 T with galvanometer G is put across any number of the tenth- 

 ohm coils, and any fraction of the slide-wire. Of course two 

 galvanometers, as shown in the diagram, are not necessary : 

 a double pole-switch puts the actual instrument into either 

 circuit as desired. 



The voltage on the ends of one of the tenth-ohms (taken 

 as unit) is then 1000- 10,000- or 100,000th part of the 

 cadmium, i. e., approximately 1000, 100, or ] microvolts, 

 according to the position of K. 



Fig. 2 represents the actual instrument. The greatest 

 care has been taken to avoid accidental therm oelectromotive 

 forces, which are the chief trouble in using thermocouples. 

 The couple I have actually used so far is constantan copper, 

 which gives about 4000 microvolts between 0° and 100°. 

 The only metals used in the measuring circuit are copper 

 and manganin. All the coils are of the latter metal ; the 

 slide-wire is of gilt manganin ; the galvanometer connexion 

 is made by a short bridge between the slide-wire and a similar 

 galvanometer wire. (These two wires are shown in the figure 

 by the side of the scale, though actually underneath it.) 

 The slider makes contact always, a separate key being used 

 to put the galvanometer in circuit. The potentiometer is 

 inclosed in a wooden case, lined with thick sheet copper ami 

 filled with paraffin-oil to keep the temperature constant, and 

 the two points where the copper thermocouple leads join the 

 manganin measuring circuit are carefully buried deep down 

 in the interior of the box and near together. 



