ON THE MODULUS OF TORSIONAL RIGIDITY OF METAL WIRES. 7 



thermometers at different distances from the plate, and the results showed that 

 the plate was much hotter than the rest of the enclosure. The seriousness of this 

 is at once evident, for not only were the temperatures which had l>een taken to be 

 those of the vibrator too low, but heat must have been continually flowing from the 

 wire, which, therefore, could not have been at the temperature of the jacket 

 enclosing it. 



It was first attempted to stop this leakage of heat from the wire by interposing a 

 rod of some non-conducting material between the vibrator and the wire. Some 

 experiments were made on the conductivities of various materials, and finally slate 

 was selected as being the best from a workable and non-conducting point of view. 

 A rod of slate, 4 inches long and inch diameter, was fitted between the vibrator 

 plate and the clamp which held the wire. Even with this arrangement the plate 

 warmed up a little, about 2 C., when the heating jacket was at 1(10 C. In order to 

 see how this affected the distribution of temperature along the wire, a copper 

 wire was suspended in the heater, and to this were soldered three nickel wires at 

 different points along its length, thus giving three pairs of thermo-electric junctions, 

 by means of which any temperature differences could be detected. The results were 

 surprising. A point, ^ inch from the lower clamp and 4 inches inside the heating 

 jacket, was found to be at a temperature 3 C. below that of the centre of the 

 wire, while at a point on the wire, 6 inches inside the heater, this difference was 

 2 '8 C. These experiments indicate that the only way to obtain a uniform tempera- 

 ture in the wire is by enclosing the whole vibrating system in the heating jacket. 

 The interposition of non-conducting material reduces the temj>erature differences, but 

 does not entirely do away with them. 



As the heat had been leaking out of the wire through the bottom clamp, it must 

 also have been doing the same through the top one. The top clamp was therefore 

 severed from the bar E (fig. I), and a piece of slate, 3 inches in length, interposed 

 between the two. The rod was then lowered further inside the heater, and the 

 difference of temperature between the upper end of the wire and the centre was 

 determined. No deflection could be obtained on the galvanometer used, showing that 

 the temperature difference was less than '01 0. After the new heater descril>ed in this 

 paper had been made, the distribution of temperature along the wire was again tested, 

 but no difference could be found between that at the centre and that at either end. The 

 piece of slate interposed in the upper clamping rod unfortunately cracked with the 

 heat after being used for a few weeks, and a similar one of marble was therefore 

 sulwtituted. This was exceedingly difficult to turn owing to its brittleness, but it 

 has successfully withstood the effects of temperature for over a year, while further 

 experiments with the thermal junctions proved it to be as effective as slate in 

 preventing a leakage of heat. 



It might here be mentioned, that this research was commenced at the Physical 

 Laboratory of the University of Birmingham. In the first form of apparatus used, 



