M. J. Nickiés on Friction and Pressure. 253 
fort to overcome it in order to make this electro-magnet move on 
its armature. 
The Treatise on General Mechanics by Morin does not men- 
tion the first relation, that of the friction of iron on iron. It is 
well known however, that in practice the number 0:33 is adopted 
or it. 2 
[have reasons for thinking that the relation is the same in 
amount between magnetic pressure and the friction of sliding 
under this new condition. If the experiments which I have made 
on this subject do not prove that the friction is equal to one-third 
the pressure, they tend at least to establish an identity of relation 
tween the two pressures and the two frictions. I have had 
occasion to recognize a like identity between the adhesion or 
friction of rolling under the influence of added weight alone and 
the adhesion or friction of rolling under the influence of magnetic 
_ attraction.* 
The electro-magnet employed in the experiment was one of 
Joule’s+ The helices consisted of 30 metres of copper wire, 2 
mm. in section ; each of its polar surfaces were 0-080 m. in length 
and 0-020 m. in breadth. They are shown in figures 1 and 2, which 
tepresent, in elevation and section, the apparatus as well as the 
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different phases in the experiments. The armature consisted of a 
€ce of a railroad rail, e, placed level on a bar o wood, s; the 
* Magnet and armature had been carefully cleaned and polished. : 
A cord attached to the axis of the magnet was connected with 
‘system of pulleys, 2, 2, and at the other extremity was attached 
* See this Journal, vol. xvi, p. 387, Nov., 1853. + Phil, Mag, [4], ii, 450. 
