A. M. Mayer on measuring Electrical Conductivities. 309 
following process. The axis of a helix, 8°7 ins. long and con- 
taining 558 feet of ;; in. copper wire, was placed in the line of 
“the dip” are a current so sent through it from ten Bunsen 
cells that its N. pole was toward the earth. The separate bars 
were then drawn through the helix until they ceased to acquire 
an increase of magnetism. This method gives a uniform and 
powerful magnetization, and probably may be improved by 
causing the bars to vibrate as they pass through the helix ; 
which can be accomplished by means . a tuning - fur- 
nished with a long brass stem. After the magnets were com- 
bined, as described above, a weight of 1°5 Ib. was is at 
the end of the middle bar. 
The magnet was supported in an EH. and W. line, 15°5 in. 
abov ve the surface of the mercury in the cavities of the wooden 
‘‘connecting-block” placed below it; and 2°5 in. of its S. end 
projected — the wooden clamp which held it. 
The were formed of ;'; inch “ double- went ” Lake 
Snpetioes wire. ach spiral contained 176-06 in. of wire coiled 
im 20 turns, and the terminals were 15% in. long, thus making 
- 207-06 in. of wire in each cance The greatest ran of the 
spirals was 3°9 in. and each had a central opening of 1°7 in. 
Their thickness after they were covered with naire Saas — 
and varnished was ‘06 in. The covering of the two terminal 
wires of each spiral was saturated with melted paraffin; they 
were then firmly tied together with silken cord to about ‘4 in, 
of —. ends where they separated and formed forked reais 
tio 
The spirals were formed in this manner. An iron plate A, 
which screws on to the mandrel of a lathe, has cemented on to 
its face a dise of hard wood b, 1°7 
in. in diameter and ‘1 in. thick. 
From the center of the plate A 
the plate B is screwed ‘home’ 
the dise 6 fits into the cavity }’ 
and the plates A and B are sepa- 
zw 
wire, while the disc 6 forms a cylinder between them on which 
to ao ae iral. 
of the wire to be coiled is passed through a hole d 
in a ae B, which is then screwed home on to A. e lathe is 
then turned so that the wire is coiled over the center dise from 
A to’. After the space between the disc is filled with coils, 
the f free end of the wire is secured and the plate B unscrewed, 
while the wire slides through d and the coil is not unwrap- 
