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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 the water.

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 extremely minute current, which cannot charge it very rapidly. This current is so small that even a rather sensitive galvanometer cannot readily detect it, unless the diameter of the paper washers is quite large and they are quite moist.


No. 3. 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 solid piece.

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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