498 



Mr. S. Bidwell. Changes 'produced by [May 19, 



column of Table I and in the middle curve of fig. 1. The latter 

 shows clearly that the maximum elongation had risen from 11*5 to 

 14*5 ten- millionth s, while the decrement in a field of 315 had fallen 

 from 22'5 to about 17"5. 



Exp. 3. — The current through the iron wire was then increased, 

 by an alteration of the rheostat, to 2 amperes. The further elonga- 

 tion of the wire due to the heating effect of the increased current 

 was very nearly 1000 ten-millionths, corresponding to a rise of tem- 

 perature of 8° *2 C. This added to 2*5, the rise due to the current of 

 1 ampere, which was passing before, gives 10°* 7 as the excess of the 

 temperature of the wire carrying 2 amperes above that of the room. 

 When the index had become steady, which happened in the course 

 of about 2| minutes, another series of observations was made ; but 

 instead of applying all the previously employed magnetising forces 

 in succession, alternate ones were omitted. This was done for the 

 purpose of shortening the experiment, it being thought doubtful 

 whether the Grove's cell which supplied current to the iron wire 

 would remain sufficiently constant when giving so strong a current 

 as 2 amperes. The results of the experiment are contained in the 

 last column of Table I, and in the highest of the curves in fig. 1. 

 There is again a marked increase of the maximum elongation, and 

 decrease of the retraction in a field of 315. 



For the sake of easy comparison, the principal results obtained 

 with this wire are collected in Table II. 



Table II. — Iron Wire, diameter - 75 mm. 



Current through 

 iron wire. 

 Amperes. 



Maximum elongation 

 in ten-millionths 

 of length. 



Retraction in field 

 of 315 C.G-.S. 

 units. 



Field in which 

 length is 

 unchanged. 







11-5 



22-5 



130 



1 



14-5 



17 '5 



170 



2 



20 



12 



200 



Exp. 4. — The previous observations were repeated with another 

 specimen of soft iron wire of greater diameter, viz. : 1*05 mm., no 

 current being at first passed through it. The results appear in the 

 second column of Table III and in fig. 2. 



within a quarter of a scale division, with the exception of the two last, which showed 

 that the E.M.F. of the "battery — seven Grove's cells — was slightly increasing, or 

 rather perhaps that its internal resistance was diminishing. When two successive 

 readings with the same resistance in circuit differed by no more than a quarter of 

 a scale division (equivalent to 3*125 units of magnetising force), the mean of the two 

 readings was taken as giving the true current. 



