1f> DR. F. HORTON ON TEE KFFECTS OF CHANGES OF TEMPERATURE 



Except in the case of aluminium and of the soft metals tin, lead, and cadmium, tin- 

 value of X was never greater than '00115, and it was generally much less than this. 

 Taking A. = '001 1 5 the value for a gold wire at 100 C. the period was 5'4 . . . seconds 



so that 



T,, = 5-4 ... (1 - '000,000,08) 



or the correction in this case is negligible. 



In the case of the soft metals it was found to be impossible to time the vibrations 

 to such a degree of accuracy as was obtained with the other wires, and only in a tt-\\ 

 instiinces was it necessary to apply the correction for damping. 



The Moments of Inertia of the Vibrator*. 



The same vibrator was used for observations on wires of copper, iron, steel, silver, 

 platinum, gold, and aluminium. It consisted, as already described, of a circular plate 

 of gunmetal screwed through its centre on to the lower end of a steel rod, the upper 

 end of which was formed into a clamp to hold the wire. The total mass of this 

 vibrator was about 1 kilogramme. Lighter vibrators had to be used in the case of the 

 softer metals. With cadmium a rectangular sheet of brass weighing 100 grammes was 

 substituted for the circular gunmetal disc. This was replaced by a similar sheet of 

 aluminium in the case of tin and of lead. The moments of inertia of these two latter 

 vibrators were found with sufficient accuracy by calculation from their masses and 

 dimensions ; but more exact knowledge was required in the case of the circular 

 vibrator on account of the greater precision with which the torsional periods of the 

 harder wires could he determined. The moment of inertia of the gunmetal vibrator 

 was therefore determined experimentally, the wire on which it was suspended being 

 a steel one. Attempts were also made to find the moment of inertia of the vibrating 

 system in the case of other wires, but it was found to be impossible to do so, for in 

 nearly all the wires investigated the rigidity was not a constant quantity, but varied 

 very much even at one temperature, so that determinations made at different times 

 could not be compared. Also in the case of the metals copper, gold, and platinum 

 the extra weight of the ring used in determining the moment of inertia altered the 

 rigidities of the wires very considerably, so that the periods with the ring on could 

 not be compared with those of the vibrator alone on this account. The experiment 

 was tried with each wire, and it was found that the time of vibration of the plate 

 alone, before the ring was put on, was largely different from that after it had been 

 taken off. The figures obtained in the determination of the moment of inertia of this 

 vibrator, and its variation with the temperature, will be given later. 



The, lengths and Radii of the Wires. 



Tin- wires experimented on were all about 58 centims. long. For the purpose of 

 measuring their lengths, a fine mark had been scribed on the upper clamping rod E 



