Learn about Life in the 1920s

How to Make Apparatus for Simple Scientific Experiments

Colour Discs for the Gramophone. - The gramophone, by virtue of its table revolving at a controllable speed, comes in useful for a series of optical experiments made with coloured discs bearing designs of different kinds.

These spinning designs can also be used for tops - simply by inserting a pointed stick like a pencil through the center to form a spindle.


The material needed for these discs is cardboard, covered with white paper on one side, or the Bristol board used by artists. The discs on which the designs are drawn should be made as large as the gramophone table will take conveniently, so as to be viewed by a number of people at once. To encourage readers who do not possess a gramophone, it may be pointed out that a gramophone, is merely a convenience, and not indispensable for turning the discs, which may be revolved on a sharpened pencil or any other spindle with pointed ends.


The Vanishing Spirals (Fig. 137). - This design, if spun slowly in a clockwise direction, gives one the impression that the lines all move in towards the centre. If the disc is turned in an anti-clockwise direction, the lines seem to move towards the circumference and disappear. To get the proper effect the gaze should be fixed and not attempt to follow the lines round.

The Rolling Circles. - Figs. 138 and 139 are variations of the same idea. In Fig. 138 two large circles are described cutting one another and enclosing a smaller circle concentric with the disc.


When spun at a certain rate the larger circles will appear to run independently round the small. The effect is heightened if the circles are given different colours. If black only is used for the large circles, the eyes should be kept half closed. In Fig. 139 two pairs of circles are described about two centres, neither of which is the centre of the disc. The pairs appear to roll independently.

The Wriggling Line (Fig. 140). - If this design is revolved at a low speed and the eye is fixed on a point, the white (or coloured) line will seem to undulate in a very extraordinary manner.


The line is made up of arcs of circles, and as the marking out is somewhat of a geometrical problem, a diagram (Fig. 141) is added to show how it is done. The dotted curves are those parts of the circles which do not enter into the design.

Begin by marking out the big circle A for the disc. The circumference of this is divided into six equal parts (chord equal to radius), and through the points of division are drawn the six lines from the centre. Describe circles aaa, each half the diameter of A. The circles bbb are then drawn from centres on the lines RRR, and with the same radius as aaa., The same centres are used for describing the circles a1 a1 a1 and b1 b1 b1, parts of which form the inner boundary of the line. The background should be blackened and the belt left white or be painted some bright colour.


FIG. 141.

Another optical illusion is afforded by Fig. 142. Two sets of circles are described about different centres, and the crescent-shaped areas between them coloured, the remainder of the disc being left white.

The disc is revolved about the centre of the white areas, and one gets the impression that the coloured parts are portions of separate discs separated by white discs.


FIG. 142. and FIG. 143.

The Magic Spokes (Fig. 143). - Place a design like this on the gramophone and let it turn at high speed. The radial lines seem but a blur. Now punch a hole one-eighth of an inch in diameter in a piece of blackened card, and, standing well away from the gramophone, apply your eye to the hole and move the card quickly to and fro. The extreme briefness of the glimpses obtained of the moving lines seems to rob them of motion, or even make them appear to be moving in the direction contrary to the actual. Instead of a single hole, one may use a number of holes punched at equal intervals round a circle, and revolve the card on the centre. If a certain speed be maintained, the spokes will appear motionless.

The substitution of a long narrow slit for a circular hole gives other effects.


FIG. 144.

A Colour Top. -- Cut a 4-inch disc out of white cardboard and blacken one-half with Indian ink. On the other half draw four series of concentric black lines, as shown in Fig. 144. If the disc is mounted on a knitting needle and spun in a horizontal plane, the black lines will appear of different colours. A clockwise rotation makes the outermost lines appear a greenish blue, those nearest the centre a dark red, and the intermediate groups yellow and green. A reversal of the motion reverses the order of the colours, the red lines now being farthest from the centre. The experiment is generally most successful by artificial light, which contains a larger proportion of red and yellow rays than does sunlight. The speed at which the top revolves affects the result considerably. It should be kept moderate, any excess tending to neutralize the colours.


FIG. 145.

The Magic Windmill. - Mark a circle 2-1/2 inches in diameter on a piece of notepaper, resting the centre leg [of the compass] so lightly that it dents without piercing the paper. With the same centre describe a 3/4-inch circle. Join the circles by eight equally spaced radial lines, and an eighth of an inch away draw dotted parallel lines, all on the same side of their fellow lines in order of rotation.

Cut out along the large circle, and then with a sharp knife follow the lines shown double in Fig. 145. This gives eight little vanes, each of which must be bent upwards to approximately the same angle round a flat ruler held with an edge on the dotted line.

Next make a dent with a lead pencil at the exact centre on the vane side, and revolve the pencil until the dent is well polished.


FIG. 146.

Hold a pin, point upwards, in the right hand, and with the left centre the mill, vanes pointing downwards, on the pin (Fig. 146). The mill will immediately commence to revolve at a steady pace, and will continue to do so indefinitely; though, if the head of the pin be stuck in, say, a piece of bread, no motion will occur.

The secret is that the heat of the hand causes a very slight upward current of warmed air, which is sufficient to make the very delicately poised windmill revolve.

A Pneumatic Puzzle. - For the very simple apparatus illustrated by Fig. 147 one needs only half a cotton reel, three pins, and a piece of glass or metal tubing which fits the hole in the reel.

Adjust a halfpenny centrally over the hole and stick the pins into the reel at three equidistant points, so that they do not quite touch the coin, and with their ends sloping slightly outwards to allow the halfpenny to fall away.


FIG. 147. -- Apparatus for illustrating an apparent scientific paradox.

Press the coin against the reel and blow hard through the tube. One would expect the coin to fall; but, on the contrary, the harder you blow the tighter will it stick, even if the reel be pointed downwards. Only when you stop blowing will it fall to the floor.

This is a very interesting experiment, and will mystify onlookers who do not understand the reason for the apparent paradox, which is this. The air blown through the reel strikes a very limited part of the nearer side of the halfpenny. In order to escape, it has to make a right-angle turn and pass between coin and reel, and, while travelling in this direction, loses most of its repulsive force. The result is that the total pressure on the underside of the coin, plus the effect of gravity, is exactly balanced by the atmospheric pressure on the outside, and the coin remains at that distance from the reel which gives equilibrium of forces. When one stops blowing, the air pressure on both sides is the same, and gravity makes the coin fall away.

The function of the pins is merely to keep the halfpenny centred on the hole. If steam is used instead of human breath, a considerable weight may be hung from the disc without dislodging it.

The Magic Swingers. - The easily made toy illustrated on p. 321 is much more interesting than would appear from the mere picture, as it demonstrates a very striking physical phenomenon, the transference of energy. If two pendulums are hung close together from a flexible support and swung, their movements influence one another in a somewhat remarkable way - the swing of the one increasing as that of the other dies down, until a certain point is reached, after which the process is reversed, and the "dying" or "dead" pendulum commences to come to life again at the expense of the other. This alternation is repeated over and over again, until all the energy of both pendulums is exhausted.


FIG. 148.- Magic pendulums.

To make the experiment more attractive, we substitute for the simplest possible pendulums - weights at the end of strings--small swings, each containing a figure sitting or standing on a seat, to the underside of which is attached a quarter of a pound of lead. To prevent the swings twisting, they are best made of strong wire bent as shown in Fig. 148, care being taken that the sides are of equal length, so that both hooks may press equally on the strings. Eighteen inches is a good length. The longer the swing, and the heavier the weight, the longer will the experiment last.

The swings are hung, six inches apart, from a stout string stretched tightly between two well-weighted chairs or between two fixed points. The string should be at least 4 feet long. With two equally long and equally weighted pendulums, the three following experiments may be carried out:-

1. Let one, A, start from rest. The other, B will gradually die, and A swing to and fro more and more violently, till B at last comes to a dead stop. Then A will die and B in turn get up speed. The energy originally imparted to B is thus transferred through the string from one pendulum to the other an indefinite number of times, with a slight loss at every alternation, until it is finally exhausted by friction.

2. Swing them in opposite directions, but start A from a higher point than B. They will each alternately lose and gain motion, but will never come to rest, and will continue to swing in opposite directions--that is, while A swings north or east B will be swinging south or west, and vice versa.

3. Start them both in the same direction, but one from a higher point than the other. There will be the same transference of energy as in (2), but neither will come to rest between alternations, and they will always swing in the same direction.

Unequal Lengths. - If for one of the original pendulums we substitute one a couple of inches longer than the other, but of the same weight, the same set of three experiments will provide six variations among them, as in each case either the longer or the shorter may be started first or given the longer initial swing, as the case may be. The results are interesting throughout, and should be noted.

Three or more Pendulums. - If the number of pendulums be increased to three or more, the length of all being the same, a fresh field for observation is opened. With an increase of number a decrease in the individual weighting is advisable, to prevent an undue sagging of the string.

In conclusion, we may remark that a strong chain stretched between two trees and a suitable supply of rope will enable the reader and his friends to carry out all the experiments on a life-size scale.

A Smoke-ring Apparatus. - Get a large tin of the self-opening kind and cut a hole 2 inches across in the bottom. Then make a neat circular hole 1-1/4 inches in diameter in the centre of a paper disc somewhat smaller than the bottom of the tin, to which it is pasted firmly on the outside.


FIG. 149. - Smoke-ring apparatus.

The other end--from which the lid is removed--must be covered with a piece of sheet rubber stretched fairly tight and secured to the tin by string passed over it behind the rim. An old cycle or motor car air tube, according to the size of the tin, will furnish the rubber needed; but new material, will cost only a few pence (Fig. 149). A dense smoke is produced by putting in the tin two small rolls of blotting paper, one soaked in hydrochloric acid, the other in strong ammonia. The rolls should not touch. To reduce corrosion of the tin by the acid, the inside should be lined with thin card.

A ring of smoke is projected from the hole in the card if the rubber diaphragm is pushed inwards. A slow, steady push makes a fat, lazy ring come out; a smart tap a thinner one, moving much faster. Absolutely still air is needed for the best effects, as


FIG. 150. - Smoke-making apparatus.

draughts make the rings lose shape very quickly and move erratically. Given good conditions, a lot of fun can be got out of the rings by shooting one through another which has expanded somewhat, or by destroying one by striking it with another, or by extinguishing a candle set up at a distance, and so on. The experimenter should notice how a vortex ring rotates in itself while moving forward, like a rubber ring being rolled along a stick.

A continuous supply of smoke can be provided by the apparatus shown in Fig. 150. The bulb of a scent spray is needed to force ammonia gas through a box, made air-tight by a rubber band round the lid, in which is a pad soaked with hydrochloric acid. The smoke formed in this box is expelled through a pipe into the ring-making box.

Caution. - When dealing with hydrochloric acid, take great care not to get it on your skin or clothes, as it is a very strong corrosive.

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List of Chapters in this book: Sawing Trestle | Joiner's Bench | Bookstand | House Ladder | Developing Sink | Poultry House | Bicycle Shed | Rifle Target | Cabinet Making | Telegraphic Apparatus | Electric Motor | Alarm Clock | Model Railway | Reciprocating Engine | Slide Valve Engine | Model Steam Turbine | Steam Tops | Model Boilers | Quick Boiling Kettle | Hot Air Engine | Water Motor | Model Pumps | Kites | Paper Gliders | Model Aeroplane | Scientific Apparatus | Rain Gauge | Wind Vanes | Strength Tester | Harmonographs | Automatic Matchbox | Wooden Workbox | Wrestling Puppets | Double Bellows | Pantograph | Silhouette Drawing Machine | Signalling Lamp | Miniature Gasworks