Fis. 8. 
94 DE. JOULE ON SOME THEEMO-DYNAiHC PEOPEEIIES OF SOLIDS. 
are parts of one sewing-needle magnetized to satui'ation, one part being a Uttle longer 
than the other, so as to exceed it in magnetic moment and gire direction to the sys e^ 
A piece of glass tube drawn very fine, and bent as represented m the sketch, is attac 
at right angles to the upper magnetic needle and serves as 
the pointer. The lower needle is hooked to the pointer by 
means of the fine glass tube to which it also is attached. 
The coil consists of twenty turns of silked copper we, 
^o-th of an inch in diameter, the ends of which dip into the 
mercury cups gg formed in the block of wood. 
8. In order still further to increase the sensibility of the 
instrument, a steel magnet one yard long, the permanency 
of which had been tested, was placed so as to counteract ^ . , , ,. . , 
and almost entirely overcome the action of the earth’s magnetism in the locahty o t e 
needle. A small telescope placed at the distance of a few ^ards, and oo ' g o ique y 
downwards through the chimney glass at the graduated circle, comp ete t le appa 
9. With air in the receiver at the atmospheric pressiu-e, the mere standing at the 
distance of two yards on one side of the instrument, would in a short space oi time 
cause the needle to travel through 10° in consequence of the cmrents of an produce 
by the unequal heating of the waUs of the glass receiver. But when the air was r educe 
to a pressure of only half an inch in the mercuiT gauge, this did not take place, tlioi^ i 
still, when the hand was put in contact with the receiver, a very considerable deflection 
of the needle was speedily produced. ^ i i 
10. On working with my instrument, I was agreeably surprised to find that ^A^hen ttie 
bar-magnet was placed so as to make the needle take up one minute in bemg deflecte 
to a new position, no perceptible return swing of the needle took place, even w len t le 
rarefaction of the air was carried to half an inch pressure. If a small magnet was sud- 
denly placed where it could deflect the needle 30°, the pointer woiild steadily traie 
towards that degree of deflection, and on arrmng there would remain settled nut lou 
any previous oscillation that could be discerned, men the time of^ snung nas 
reduced to 30", a return swing was observed amounting to yio, and ^ll^^ 
swing, according as the gauge was reduced to 1, and \ inch respectively. 
11. As a test of the delicacy of the instrument arranged so as to give the snung in 
45" I may mention that, after increasing the resistance of the coil and its appen ages 
a hundredfold by the addition of 100 yards of fine copper wire of -jotk ol an uicii 
diameter, a single degree Centigrade communicated to the junction bismuth and anti- 
mony produced a deflection of 2° 57'. Therefore, as it was quite possible to eskina e 
deflection amounting to 2', it followed that a change of temperature in the jiiiic ion 
bismuth and antimony directly connected with the multiplier would be estimable, it it 
were only -gwo of a degree Centigrade. i owoinU- 
12. An objection which might be raised on account of an air-pump being pei i ) 
