

Thermo-electric Powers of Metals and Alloys. 103 



arrangement (see PI. III. fig. 2). The calibrated manganin 

 wire used for the resistance-balance serves also as a poten- 

 tiometer wire, and the same secondary battery and galvano- 

 meter by a different arrangement of connexions are employed 

 to complete the potentiometer. A platinoid resistance of 

 18,200 ohms was constructed, and also a set of four coils of 

 manganin wire of the same diameter as the bridge wire were 

 prepared, and the whole of these coils joined in series with the 

 bare manganin wire stretched over the scale. These four 

 coils had resistances respectively equal to one, two, three, and 

 four times that of the bridge slide- wire. We then con- 

 structed an arrangement by which we could use the same slide 

 wire either as a bridge to measure the resistance of the work- 

 ing platinum thermometer, and hence to determine the 

 temperature of the junctions inside it, or to measure the 

 electromotive force set up in the thermo-couple by balancing 

 it against a variable standard potential-difference calibrated 

 with reference to a Clark cell. 



It was necessary to make provision for changing the bridge 

 into a potentiometer very rapidly, and for the purpose the 

 connexions of the different parts of the apparatus were 

 arranged as shown in PI. III. fig. 2, so that by one movement 

 of a double copper connecting-piece the connexions of the 

 apparatus could be instantly changed from a bridge to measure 

 the temperature of the thermometer, to a potentiometer to 

 measure the electromotive force of the thermo-couple. The 

 whole of the electromotive-force measurements are therefore 

 referred to a Clark cell. 



The arrangement of the combined bridge and potentiometer 

 may be described a little more in detail as follows, and will be 

 easily understood by reference to the diagram in PL III. fig. 2. 



XZWY is the two-metre slide- wire having its ends attached 

 to terminal blocks X and Y. B is the two- cell battery which 

 is connected through the current-reverser D with the ends of 

 the slide-wire, having interposed between one terminal and 

 the block Y either a resistance R 2 of about 200 ohms, when 

 used as a bridge, or resistance R l5 R 4 , and R 3 when used as 

 a potentiometer. R x was a variable resistance of about or up 

 to 5000 ohms; R 4 was a resistance of 18,200 ohms; and R 3 a 

 resistance of 10x25*576 ohms divided into four sections 

 having ratios of 1, 2, 3, and 4 in magnitude. The galvano- 

 meter G was connected to terminals U and V when used as a 

 bridge, and P was the slide contact on the wire ; the other 

 two arms consisted of the platinum thermometer P x joined 

 in between n and o and the 5-ohm coil C joined in between / 

 and m. When used as a potentiometer, the thermo-couple Th 



