to the Passage of an Electric Current. 



407 



clear note to be got out of the wire by rubbing it longitudinally with a 

 resined glove. The wire was then clipped in the centre and rubbed again 

 to test the security of the fastenings, as it was found that if the bar in 

 the vice was not well secured the note produced by clipping in the centre 

 was not the octave of the note given out by the undipped wire, but if 

 the bar were well fastened a clear and perfect octave could be produced. 



The number of longitudinal vibrations was ascertained by means of the 

 siren, generally three or four trials of 3 minutes each being taken. In 

 this work I was assisted by Mr. Purse (the assistant to the Professor of 

 Natural Philosophy at King's College), to whom I am much indebted for 

 help in my experiments. The records of the siren were very good 

 indeed, in every case there never being a greater difference than ^ per 

 cent., and in many cases less. But on experimenting with different 

 lengths of the wires the values of the modulus would vary as much as 

 1 or 2 per cent., so that, though some of the values obtained are recorded, 

 I have chosen the more reliable ones of the cathetometer for calculating 

 the increase of length of each wire when stretched. 



In the following Table the amount of lengthening which a cubic centi- 

 metre of each wire experiences for a stretching -force of 1 gramme is 



given ; these values will be denoted by — : — 



e 



Values of — obtained from Values of — obtained from 



e e 



observations with the observations on longi- 



cathetometer. tudinal vibrations. 



Steel. 



(1) — * 4866 xlO- 13 



(2) 5279xl0-i 3 4928x10-13 



(3) 5082x10-13 5073x10-13 



(4) -* 



(5) 5665x10-13 



Iron. 



(1) 4896x10-13 



(2) 5938x10-13 



(3) 5435x10-13 



Brass. 



(1) 10120x10-13 



In the case of those wires marked * there was not sufficient length to 

 take good observations with the cathetometer. 



Again dividing the values of — by those of — we shall obtain the in- 



s e 



crease of resistance per unit of resistance per unit increase of length. 



VOL. XXVI. 2 F 



