Interesting Experiments in Electricity
By P. Mertz
No. 1. An Electric Pump
This pump is a purely electromagnetic
one, and although it is extremely
weak, nevertheless it is
very interesting as a curiosity.
Referring to Fig. 1, A is a tin can; almost
any size will do very well. In the
bottom of the can is put a disc (B) of
mica, celluloid, hard rubber or any insulator
which is not affected by being immersed
in water. C is as powerful a bar
magnet as can be procured; it is better
if it is of circular cross-section, as it will
not interfere so much with the motion of
A solution of copper sulphate in water
is to be poured into the can. The terminals
of a fairly powerful source of current
(D) are then connected, one to the
tin can (A) and the other to the bar magnet
(C). As soon as the current is turned
on the water will be observed to begin to
turn around the bar magnet. If little
pieces of paper, wood, etc., are dropped
in the water the motion will be more
easily seen. If the direction of the current
in the water is reversed the latter will
turn in the opposite direction. With a
strong bar magnet and current the water
will rotate at a pretty good speed. If the
water rotates very slowly, or not at all,
due to the current not being strong enough,
this can be remedied by using a can of
less diameter, so that there will be less
water to move.
Although this apparatus is hardly worthy
of its name as a pump, it is easily
seen the centrifugal action of the
water as it rotates in the can suggests a
somewhat more real pump.
The motion of the water can easily be
explained from the diagram, Fig. 2. The
current as it crosses from A to C encounters
the magnetic lines of force of
the magnet in such a way as to produce
motion around the latter.
No. 2. A 500-Volt Dry Cell.
This dry cell, although having such an
enormously high voltage, nevertheless occupies
a space of only about 8 x 1 1/2 x 1 1/2
inches, and is as dry as the atmosphere
which surrounds it.
It is made of a great number of thin
paper washers about 1 1/2 inches in diameter,
painted on one side with aluminum
paint and on the other with gold paint.
As illustrated in Fig. 3, they are strung
on a 1/4 inch hard rubber rod, great care
being taken to see that the aluminum sides
of all the washers face one direction and
the gold sides the other. The washers are
compressed together between two metal
washers, each under a nut screwed over
the end of the hard rubber rod.
This forms a voltaic pile, the aluminum
and copper faces, respectively, acting like
the more usual zinc and copper discs, and
the paper with the slight trace of moisture
it gathers from the surrounding atmosphere,
as the usual cloth wet with acidulated water. The extremely small amount
of moisture is quite sufficient for generating
the extraordinarily high (for a dry
cell, at least) potential. The voltage of
the cell can easily be tested if one end
of it is grounded and the other touched
to an electroscope. It will be noted, however,
that the electroscope will take some
time before deflecting, on account of the
minute current, which cannot
charge it very rapidly. This current is so
small that even a rather
cannot readily detect
it, unless the diameter
of the paper washers
is quite large and they
are quite moist.
The Electric Snake.
This is simply an
iron tripod stand of
the kind used in physics
experiments (as shown
in the illustration, Fig. 4), over which
a solenoid has been slipped so as to
make a magnet of it. At the top end
a clamp is fastened to hold one end of
a very flexible conducting cord, the other
end of which is fastened to a screw, or
whatever is handiest, on the top wooden
end of the solenoid. The cord is connected
in series with the solenoid with a fairly
strong current, and a reversing switch is
inserted in such a way as to be able to
reverse the current in the cord without
affecting that in the coil. By turning the
switch first one way and then the other
the cord will coil around the iron support
in a way strongly suggesting a snake. The
current, however, has to be fairly strong
to make this experiment successful.
No. 4. Experiments with Induction in
an Aluminum Ring.
If a light aluminum ring is slipped over
one end of a fairly powerful electromagnet,
such as is illustrated in Fig. 5, when
the magnet is energized the ring will suddenly
take a jump either toward or away
from the coil. If then the current is reversed
the ring will jump in the opposite
direction. If the magnet is held vertically,
instead of horizontally, the ring will be
able to jump in only one direction (i. e.,
upward), but the length of jump will be
increased, due to lack of friction. Indeed,
if a strong enough magnet is used the ring
will have no difficulty in jumping right off
the magnet and landing on the floor. The
experiment will be far more striking if
the core of the magnet is made of a bundle
of iron wires than if it is made of a
If the magnet is energized with alternating
current the ring will float in a certain
position without apparent support. If
disturbed from that position it will very
gently and gracefully come back to it.
These experiments are especially mysterious
to most because the ring is of
aluminum, a non-magnetic material. The
explanation, however, is simple; it is
merely that a strong current (due to its
low resistance) is induced in the ring, and
this current generates a magnetic field,
which, reacting with the field of the magnet,
produces the motion.
No. 5. The Magic Light.
On the table are intricate apparatus resembling
a wireless set and a Tesla coil.
A few feet away is another Tesla coil,
minus its heavy coil of wire, the fine coil
alone remaining. The two ends of this
coil are connected to a very small pea incandescent
lamp, not lighted.
The performer turns on the switches;
still the lamp does not light. He says a
few occult words and waves his hands
mysteriously around, and, lo and behold,
it lights up! With a few more magic
words and waves of his hands he extinguishes
it. He then offers to blow it on.
He first blows hard at it, but it does not
light up; he comes nearer, blows very
gently and, yielding to his coaxing, the
lamp lights up! He blows again and the
light disappears from the lamp.
The electrical man in the audience is
much peeved. "There are connections under
the carpet," he says. The performer
offers to try the experiment standing on
a big board placed on the floor. He then
does the same things, without the slightest
lessening of his magic powers.
Various members of the audience offer
more or less ridiculous explanations, but
no one guesses the true one.
How did he do it? It is very simple.
He had a Tesla coil rigged up with a tuning
helix. The other Tesla coil was connected
to the lamp, and one terminal of
each coil was grounded. Just before the
experiment the performer tuned the transmitter
until the lamp brightened up, and
then put in a trifle more inductance, just
enough so that the lamp would be extinguished.
Then he placed his hand near
the receiver. The result of this was that
the lamp circuit had more capacity than
before, which raised its wave length and
brought it in tune with the transmitter,
lighting the lamp. When he waved his hands
he brought them near the coil, and when
he blew at the lamp he brought his head
near the same coil. When he extinguished
the lamp he merely moved away from the
coil. No one ever guesses that the proximity
of some part of his body to the coil
has anything to do with lighting or extinguishing
the lamp. When he got on
the board he still had enough capacity to
produce the desired effect.
The whole thing is so simple that any
wireless experimenter who has a transmitting
set can easily repeat it.
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