406 



Mr. H. Tomlinson on the Increase in Resistance 



(1) 2229-2 xlO- 12 



(2) 2211-8 xlO- 12 



(3) 2101-5 xlO- 12 



(4) 2435-0 xlO- 12 



Mean=2244-9 x 10~ 12 . Greatest de- 

 parture from the mean about 8*5 per 

 cent. 



It will be seen from the last Table that the values of — are as constant 



s 



as could be expected in the case of wires of the same material, but differ 

 for wires of different materials, being greatest for brass. 



The value of Young's modulus for some of the wires was ascertained 

 in the usual manner with the cathetometer. The wires were firmly 

 secured at the top of a staircase, and a mark placed about 3 feet above 

 the point of attachment of the weights at the bottom, so that the stretch- 

 ing of about 21 feet of the wire could be observed and the increase of 

 length for different weights observed. A mark was also made near the 

 top of the wire, in order to examine if there was any yielding of the 

 support to which the upper end of the wire was attached, and for some 

 of the wires a reading was taken both at the top and bottom marks ; but 

 as after several trials there was no apparent yielding, in the last experi- 

 ments made the mark only at the bottom was observed. "Weights 

 heavier than those intended to be used were first put on and then taken 

 off again several times ; finally a weight was left on the wire sufficient to 

 keep it perfectly straight. Then weights were carefully added, the in- 

 crease of length observed with the cathetometer ; and the weights having 

 been taken off, another reading taken, and if there was any slight diffe- 

 rence in the readings before adding the weight and after taking it off, 

 the mean of the two readings was taken. The results agreed very fairly 

 with each other — how much so may be gathered from the following 

 example (two trials with each weight were taken) : — 



Iron (No. 2). 



For a stretching-weight of A lengthening wag observed of 



10220 grammes -0005774 m.m. per gramme. 



7710 „ -0005837 „ 



17930 -0005828 „ 



Mean .... -0005813 „ 



The result may be taken as a fair average of the values obtained with 

 the other wires. 



The value of Young's modulus was also ascertained by means of 

 longitudinal vibrations of sound. The two ends of the wire to be 

 examined were securely soldered into two strong flat iron bars, one of 

 the bars was fastened in a vice, firmly secured, whilst the other bar was 

 held in the hand, and the wire pulled with sufficient force to enable a 



