Encyclopaedia Britannica > Volume 8, DIA-England

626
Phenome¬
na and
Laws.
Inflamma¬
tion of bo¬
dies.
ELECTRICITY.
Mr Snow
Harris’s
instrument
*br measur¬
ing the
evolution
of heat.
Plate
CCXIY.
Fig, 1, 2.
lower end of the wire, touch the wick with the knob of
the jar, and the candle will be lighted.
Exp. 3. To inflame phosphorus, fyc. Having placed
powdered phosphorus, rosin, or camphor, on some cotton
wool, and wrapped it round one of the knobs of a discharg¬
ing rod, apply the knob thus covered to the ball of a
charged jar, and the naked knob to the external coating
of the jar, and the powder will be set on fire.
Powdered rosin floating on water may be set on fire by
transmitting a charge over the surface of the water be¬
tween two points.
Exp. 4. To inflame alcohol or ether. The alcohol or
ether being placed in an insulated metallic cup, electrify the
cup, and upon taking a spark from the cup either with the
knuckle or any other conductor, the fluid will be set on fire.
If ether is placed in a thin stratum upon the surface of
water, in a clean wine glass, a spark taken from the surface
will inflame the ether when the water is connected with
the prime conductor.
Exp. 5. To inflame gunpowder. Upon the end of an
insulated metallic wire fix a small cartridge, and when
the cartridge is presented to the knob of a charged jar, the
powder which it contains will be exploded.
Exp. 6. To exhibit the heat evolved by electricity. Take
a wooden rod, for example one of red fir, about one inch
thick and ten inches long, and place it between the ball
of the conductor and the conducting wire ; put the ball of
a thermometer in a hole bored in the wood, and in a few
minutes the mercury will rise to about 112°. Van Marum,
who made this experiment, found that in three minutes the
mercury rose from 61° to 88°, and in five minutes to 112°.
The evolution of heat by electricity is finely shown by
means of a beautiful and delicate instrument constructed
by Mr Snow Harris. Mr Children and other philosophers
had deduced from a variety of facts that the heat evolved
by a metallic wire while transmitting an electric charge, is
in some inverse ratio of its conducting power; and hence
Mr Harris was desirous of measuring the relative degrees
of heat so evolved by various metals and alloys in a gaseous
medium such as air, and thus to discover their precise re¬
lations as conductors of electricity. The instrument which
he used for this purpose is represented in Plate CCXIV.
fig 1, and is little more than an air thermometer, the me¬
tallic wire to be examined being made to pass air-tight
through the ball. A glass tube CDA, whose bore is re¬
gular, and somewhat less than one tenth of an inch, has
one of its extremities DA bent upwards and outwards for
about two inches, and is united by welding to a spherical Phenoi
reservoir A, containing a coloured fluid.1 This tube is naan
fixed to a correctly divided scale E, supported by a suit- Ijawi
able base ; and the zero of the scale is at o, on a level with
the coloured fluid in the reservoir A. Above the reser¬
voir A is screw ed air-tight, by means of brass caps closely
cemented, a glass ball B, three inches in diameter; and
through this ball a metallic wire m, n, varying from yJ-th
to jth of an inch in diameter, may be passed air-tight by
means of small flanges of brass m, n, fig 2, cemented in
and round two holes drilled through the ends, each flanch
having a small projecting shoulder to receive the wire,
and upon wdiich are screwed two brass balls a, b, so as to
render the whole air-tight. In order to fix the wire, the
brass parts are made quite clean internally, and the wire
being passed directly through them, is gently stretched,
and then compressed in the holes by small pegs of tough
wood, so as to insure a good contact. The pegs and the
wire are allowed to project a little, to enable the observer
to substitute different wires expeditiously. When an
electrical explosion of sufficient force is now made to tra¬
verse the wire m, n in the ball B, the heat which it evolves
will be made evident by the ascent of the coloured fluid
along the scale E.
Mr Harris now submitted to examination equal wires
of different metals ; and in order to insure the transmission
of equal and similar explosions through each of them, he
adopted the following contrivance. Two equal brass balls
were fixed at a given distance, as in Lane’s discharging elec¬
trometer. One of them, which was insulated, was placed
in immediate connection with the positive side of the bat¬
tery, while the other was connected with the negative side;
the metallic wire to be examined forming part of the cir¬
cuit. This last connection was made by means of two fixed
copper wires inserted into the balls on each side of the
glass, and made perfect at the points of junction. When
the charge therefore of the battery was sufficiently intense
to pass the given interval, the discharge took place through
the wire in the ball. Mr Harris used a battery of Jive
jars, each containing square feet of coated surface.
They were placed on a metallic base communicating with
the negative conductor, and were charged by means oflong
copper rods projecting immediately from the bottom of
each jar. The machine employed was a plate one, with a
disc of glass three feet in diameter.
The results which Mr Harris obtained from an exten¬
sive series of experiments are given in the following table
Metals.
Copper
Silver
Gold
Zinc
Platinum
Iron
Tin
Lead
Brass
Gold 1 part, copper 1 part..,
Gold 3 parts, copper 1 part.
Gold 1 part, copper 3 parts.
Effects.
Metals.
6
6
9
18
30
30
36
72
18
20
25
15
^ (
f-5
Copper 1 part, silver 1 part...
Copper 1 part, silver 3 parts.
Copper 3 parts, silver 1 part.
Gold 1 part, silver 1 part
Gold 1 part, silver 3 parts....,
Gold 3 parts, silver 1 part....,
Tin 1 part, lead 1 part
Tin 3 parts, lead 1 part
Tin 1 part, lead 3 parts........
Tin l part, zinc 1 part
Tin 3 parts, zinc 1 part
v Copper 8 parts, tin 1 part.
Effects
6
6
6
20
J5
25
54
45
63
27
32
18
Mr Snd
Harris’
experi¬
ments (
the evo
tion off
by eled
city.
Considering the heat to be in the inverse ratio of the heats evolved from silver and copper are alike, and also
conducting power, it appears from this table, ls£, That the those from iron and platinum, and from zinc and brass,
1 Ibis fluid may consist of rectified alcohol, one part distilled water, three parts coloured tincture of cochineal, with a little sul¬
phuric acid to make the whole sour.