on the Length and the Resistance of Iron Bars. 453 



steel axis was a plane mirror, in which the image of a scale could 

 be read off by means of a telescope and cross wires. The mag- 

 netizing spirals were those belonging to a large Kleiner's electro- 

 magnet — two of 53 millims., two of 75 millims. diameter, and 

 each 23 cubic centims. in length. In case no other fixing was 

 required , they were so arranged that the two outside spirals (the 

 narrow ones) were flush with the ends of the bar, while the two 

 inner ones (the wider) were at equal distances from each other 

 and from the outer ones. The scale was so divided that one di- 

 vision corresponded to 1*270 millim., while the distance of the 

 scale from the steel axis was 3220 millims. Hence a displace- 

 ment of one division of the scale corresponded to an angle of 

 rotation of the mirror of 40"*71. The radius of the steel axis, 

 calculated from the weight and specific gravity of the wire and from 

 its length, =0*44072 millim. ; so that a motion over one division 

 corresponded to an increase in length of the bar of 0*00008698 

 millim. The magnetizing current was each time arranged in 

 such a manner that very nearly the maximum magnetizing effect 

 was obtained. The numbers given below for the elongations 

 are the means of at least six observations in each case, which 

 always agreed very closely with each other. 



From the length and width of my spirals, it might have been ex- 

 pected that small motions of the bars out of the axis of the spirals 

 would have but little influence on the apparent change in length, 

 which occurred in consequence of the tendency of the bar to go 

 to the axis of the spiral. In order to settle this point I observed 

 these changes, first when the bar was centred as completely as 

 possible, then when all the spirals were placed obliquely, again 

 when the bar almost touched their inner coils, and finally when 

 it was again well centred. The bar was a cylinder 1242 mil- 

 lims. in length and 12 millims. in thickness. 



The elongation was — 



Bar well centred • 11*7 divisions 



Spirals oblique 11*6 „ 



Spirals almost touching the bar 10*3 „ 



On repeatedly opening and closing the current .9*1 „ 



8-8 „ 



8-5 „ 



8-2 „ 



7-0 „ 



Bar again centred 11*6 „ 



Hence any appreciable influence of the measurements could only 

 be due to a very considerable eccentricity, which could not arise 

 from an accidental displacement. That the readings are less in 



