Twist on the Thermo-electric Qualities of Iron. 225 



inner piece had a pyramidal hollow, into which there fitted 

 accurately a solid pyramid (of course truncated). This latter 

 consisted of two equal wedges, which had grooves, forming- 

 together a smnll hole along the axis of the torsion-head, and. 

 of a calibre just sufficient to allow the. end of the experimental 

 wire to pass through. A strong screw was to be driven 

 against the bases of the two wedges, which, on being thus 

 pushed from behind into the pyramidal hollow, embraced the 

 end of the wire and held it firmly in position. All was 

 made of brass. Thus the trouble of soldering once in each 

 experiment the end of the wire to the inner piece was 

 obviated, but at the same time the wire was laterally and 

 considerably compressed— a thing to be avoided as much as 

 possible, for though the portion compressed was just next to 

 the uncompressed, yet a difference of temperature between 

 them (exposed and unexposed to the air as they were) might 

 produce disturbance. 



The outer piece of the torsion-head had a shallow trian- 

 gular groove, cut parallel to its axis and on its outer surface. 

 One end of the unaffected part of the experimental wire was 

 there put in, and secured in position by a number of turns of 

 wire. 



Rigidly connected with the inner piece there was a cir- 

 cular disk of very hard wood provided with ten notches round 

 its periphery. These notches served the double purpose of 

 counting the amount of twist applied and of keeping the inner 

 piece so turned in position by means of a side ratchet. It 

 was this disk that was taken hold of in twisting, and thus 

 the metallic parts were entirely free from possible disturbances 

 due to contact with the hands. 



The junction at the other free end of the part to be twisted 

 was effected by means of another arrangement very similar 

 to the last in construction, except that the outer and inner 

 pieces were now made a sliding pair in order to allow for the 

 stretching of the whole wire when loaded, and that the end 

 of it to be joined was here put between the two wedges side 

 by side with an end of another part which was to be left 

 unaffected. 



The two pieces which formed the junctions were firmly 

 mounted on a very strong block of wood at a distance of 

 about 70 cm. from each other, care being taken to have them 

 well centered in order to avoid the production of bends near 

 the clamped ends in stretching the wire with weights. This 

 centering and necessary adjustments were done carefully 

 once for all, for though I had no time to demonstrate, yet it 

 was surmised that if any bends were formed in the initial 

 act of stretching, and were further subject to frequent 



