AND STRAIN ON THE ACTION OE PHYSICAL FORCES. 
137 
or rod placed inside. The wire forming the coil had a resistance of 1782 ohm at the 
temperature of 15° C., and was distributed in 814 turns in a length of 97 centims. The 
inner diameter of the coil was 3 centims. and the outer diameter 9’0 centims., so that 
the magnetizing force at the centre would be 477 X c X 90‘4 X ‘851 , and that of the 
two extremities 47r X c X 90‘6 X '478 absolute units; consequently the average force 
throughout the coil would be 47rXcX 90‘4 X '664. As with coil A, the whole of the 
wire was well soaked in paraffin wax. before winding, and the insulation between one 
layer and another was, by determining the resistance before and after winding, 
ascertained to be quite perfect in both A and B. 
The Effect of Temporary Stress on the Alteration of Electrical 
Resistance produced by Magnetism.* 
In order to test the effect of temporary and permanent extension on the change of 
electrical resistance produced in iron wire by magnetism, the following arrangement 
was adopted :—The wire, which had a total length of from 4 to 5 feet, was firmly 
secured at one end (fig. 19, C) to a strong upright, and was then slipped through two 
Fig. 19. 
, glass tubes of small diameter about 13 inches in length; the glass tubes were placed 
! inside two copper vessels, S S, of the same length as themselves. These vessels each 
| consisted of two concentric cylinders connected by soldering at the two extremities by 
copper rings and provided with small pipes, so that cold water could be kept running 
through the space between the cylinders. The outer of the cylinders was 1 inch nearly 
in external diameter, and the inner \ inch in diameter; one of the copper vessels was 
placed inside the magnetizing coil, A, whilst the other was supported on the table. 
* It should be mentioned here that in all these experiments the magnetizing coils were placed in a 
direction j perpendicular to the magnetic meridian. 
MDCCCLXXXIII. T 
