Electro-Magnetic Apparatus and Experiments. - 261 
that primitive current. By derivation is meant, the addition of 
another wire to any portion of the primitive circuit, The simple 
solution of the fact is, that derivation, or the addition of another 
wire, increases the conducting power of the circuit. Professor J. 
Henry’ s discovery of the method of increasing the power of the 
electro-magnet by winding upon it several short coils of wire, is 
a most striking practical illustration of this law. M. Pouillet has 
also arrived at the conclusion, that the” intensity of the current 
produced by a single element, is in an inverse proportion to. the 
real length of the circuit, The adoption of the several strands 
in the galvanometer seems therefore to be plainly indicated, and . 
experiment fully warrants it. 5, fig. 2, isa graduated circle of 
ivory for marking the deviations of the needle. _ Since the con- 
struction of the instrument, fig. 2, I have adopted the plan rep- 
resented in fig. 3, which is much to be preferred on account of 
its simplicity of construction, and the perfect | steadiness of the 
needle. ¢, is the coil cemented upon the stand d; b,a graduated 
- gone surrounding the coil. p and-», the wire terminations. a, 
the circular needle of watch spring, with a very delicate upper 
bearing at c, and a slender pivot at a, resting upon an agate centre 
cemented to the coil. As this needle is not liable to any mechan- 
~ ical displacement, it may come very near the coile. The por- 
tion of the circle between the two lines. at a, which bears the 
ret, is of brass. a 
Double Ae Sor Inducing Magnetism. 
Figure 4, represents an apparatus contrived January Lith, 1838, 
for exhibiting the magnetic forces of the centre of the helix. a, a, 
are the two helices of five layers of wire, protected by brass ca- 
sings, (split on the under side,) and by ivory heads, ¢,¢,¢,¢. b, b, 
are two curved bars of soft iron which slide readily into the he- 
lices. 0,0, the handles for pulling, furnished with ball and socket 
joints at 0, 0, to prevent the magnets being twisted or wrenched. 
The wire terminations of the helices pass through the openings 
in the brass casings, underneath the base board, and are soldered 
‘to the serew cups p, 7, for battery connexions. The attractive 
force manifested by this arrangement in the centre of the helices, 
is much greater than when an armature is applied at the extrem- 
ities. A small apparatus of this kind will resist the ‘Strength of 
two stout men pulling by the handles. This makes a very pretty 
