I do not think it need be supposed that even an amateur will buy or make a 4" or 6" coil, without having some set purpose in view, or a good idea of the uses to which he intends to put the induction coil

There are many amateurs who build or buy coils, giving a spark ranging from 1-4" to 1" with very little notion of the possibilities of such coils, and it is for these readers that I describe the following simple experiments : - A small coil experiment, which is one of the most striking and often the only one which beginners perform for the edification of themselves and friends, is a display of vacuum tubes. Current from the secondary of a 1-4" spark coil will light up tubes 6" of 8" long; but the best effects are obtained from a little larger coil than this. It may, perhaps, be better to explain here that in all experiments with high tension electricity the operator must be most particular not to be included in the path of discharge, or the result may be harmful.

Fig. 1.

A very fine experiment with a small vacuum tube and say a 1-4" spark coil, is the following : - Put the coil in action, discharging sparks as usual. Take a vacuum tube in one hand, and place the loop at the end in contact with the positive discharge rod; then gradually bring the index finger of the other hand towards the other discharger rod terminal; but on no account touch it, and tire tube will be seen to be lighted up, Another variation with these Geissler tubes is to fasten them securely in a vacuum tube rotator. Current is conveyed to the tube while the rotator is in action, giving the effect of a wheel of light. Do not use small tubes with large coils giving over 1" spark unless the platinum electrodes are very thick; but tubes of more than 6" are fairly safe on larger coils, though no definite rules can be given.

Here are a few of the more interesting experiments with the spark itself. Place a sheet of tinfoil, or a piece of glass, in contact with one of the secondary terminals. Bend a short length of wire attached to the other terminal until it is within striking distance of the tinfoil plate (Fig. 1.) Set the coil in action, and you will see the stream of sparks break into several little rays and wind across the tinfoil. This class of experiment is very numerous, and the amateur, when once started, will have much enjoyment from experi-

Fig. 2.

menting with the spark itself. Deflagration experi-ments are easy. Get two short lengths of iron wire, and put one in each end of the discharging rods.. Arrange the distance between so that the iron becomes white hot when the coil is in use. Bright sparks will be emitted from the white hot metal. Try this with different metals, and notice the difference in the color with using different wires. Let the spark pass through fine metal filings placed on an insulator. Some of the filings will be fired by the spark, and the latter in its zig-zag path will be colored according to whatever metal was used. Tip each discharger point with a little oil, and bring them very close together. The spark will be a vivid green. Lycopodium powder on cotton wool, if placed in the way of the spark, is fired; and the same thing can be done with gunpowder in very small quantities.

These will no doubt serve to start the amateur on the road to discovering many more such experiments for himself. All experiments performable with a Wimshurst machine can be done on an induction coil. Firstly, I will show how to charge a Leyden jar from a coil. Put the jar on a piece of glass, or similarly good insulator (Fig. 2.) Connect the knob of the jar to one of the discharger points, and let the other point be held at a little distance from the outer covering of the jar. When the coil is put in action a static charge of electricity will be given to the jar. This leads us to another little variation. Set the coil ready for sparking, and in addition, attach a wire from one terminal of the secondary to the knob of the jar, and a wire from the other terminal to the outer covering. When the coil is discharging sparks, the jar receives a static charge, and forming as it does an extra condenser, it

Fig. 3 discharges itself across the spark-gap together with the ordinary coil discharge. The length of the spark is thus increased.

The following is an experiment known as the "Chimes," the construction of which is shown in Fig. 3. The centre bell is connected to the ground by a

Fig. 4.

chain from its centre touching the table. When a charge is given this apparatus, the clappers are attracted to the outer bells, which are charged with electricity. When the clappers touch, they too are charged, and are repelled with enough force to enable them to both strike the centre bell. This discharges them again, and the movemenc is repeated.

Another may be styled the " Birds and Sportsman " experiment. On a small base is erected a Leyden jar ot the kind I have depicted. A strip of tinfoil is fastened along the baseboard, and the jar rests on this (Fig. 4.) The birds are of paper, and fastened to one brass ball with a piece of thin cotton. The sportsman's gun is a piece of brass wire, and is connected with the tinfoil on the base. The gun points to the other brass ball. When the jar is charged the birds will fly apart by repulsion, and remain flying for a short time. Now gradually push the sportsman nearer until his gun is very near the ball. This will discharge the jar with a loud crack, and the birds will fall. Finally, I think every amateur electrician will find that there is hardly any other instrument which he will obtain that will give such lasting pleasure as an induction coil.