This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.

Wm. B. Eddy

The aim of this paper is to show the application of the principles of induction coil design, taking for illustration a 12-in. coil. It is not to be expected that an amateur without experience or a study of the work of others should be able to design an effective and efficient coil, since there is no definite and fixed course that can be followed to obtain a desired result. Therefore, in many places it is necessary to accept the experience of others as well as using "cut and try" methods.

The design below is for a good all round coil for laboratory use, one that will give a good spark with a small amount of current, and which will also deliver a comparatively large amount of power when required; a coil that can easily produce a 12-in. spark.

We will begin the design for such. The core will be made 2 1/8 in. diameter by 24 in. long, composed of a bundle of No. 22 iron wire well annealed. Several writers advise a core 1 1/2x19 in. for a 12-in. coil, but a more powerful field than such a core can produce will be obtained from the core selected. The diameter of the core selected compared to its length is about 1 to 11 1/3, which is a good ratio for such a coil, and will allow the use of a high speed interrupter when desired.

The primary will be wound with about two and a half layers of No. 12 double-cotton-covered wire. This will give the maximum number of ampere turns which will probably ever be required and the winding for the last of only a half layer will enable one to bring out the taps without inconvenience; the extra flexibility of this arrangement compensating for the space wasted at the end of the half layer.

From wire tables it is found that there can be wound about 10.4 turns of No. 12 double-cotton-covered wire to the inch, so if an inch of space be allowed at ■each end of the core 230 turns can be wound in the remaining 22 inches. The layers will be made continuous by bringing out a connection at the end of each layer and tapping the second at the end of 30 turns and winding only 165 turns for the third layer. Two thicknesses of impregnated paper between layers will be sufficient for insulation.

Fig. 1.

The sketch of Fig. 1 shows diagramatically the arrangement of these layers and taps; the variable inductance feature may be worked with either switches or plugs to obtain any one of the 8 combinations. These connections cut into the primary circuit the following numbers of primary turns-165, 200, 230, 260, 365, 395, 460 or 625.

Before determining the thickness of the insulating tube it is necessary to find the voltage that this tube must withstand.

Curve A of Fig. 2 shows the ratio of spark length to voltage in air between needle gaps as determined and used in high-potential transformer testing; but this voltage is as read by an alternating-current instrument and is based on a sine wave, the meter registering "root mean square" values. The ratio of the maximum is to this effective value for a sine wave as 1:1/12 Since it is the maximum value which determines the sparking distance, curve B has been drawn to show

Fig. 2. these maximum values. The abscissas of this curva represents potentials and the ordinates represent sparking distances and the potential in kilovolts can be read off directly from the curve for any spark length in inches.

It will be seen from curve B that the coil which must give at least a 12-in. spark must withstand a potential of about 170,000 volts. Experience has shown that it is necessary to insulate for only 3/4 of the maximum spark length, which in this case would mean about 130 kilovolts.

Hard rubber, or ebonite, will be selected for the material for the insulating tube, and from Table I it is seen that the rupturing voltage for this substance varies from 900 to 1500 volts per inch of thickness. If the lowest value given be used and a factor of safety of 4 be taken on a safe value of 175 kilovolts per inch, a thickness of 3/4 in. will be required for the insulating tube. The length of this tube for a 24-in. core should be at least 26 in.

The inside diameter should be such that the primary and the core will fit snugly inside of it. The layers of the primary, even with the insulating paper between them, will fit into each other to a certain extent, each turn falling between two turns of the adjacent layer; therefore if the paper is .006 thick and two thicknesse3 be put between each layer and the next and a half dozen or more between the primary and the core, at least 5/8 in. inside diameter must be allowed for the primary. This will give 2 3/4 in. for the inside and 3 1/2 in. for the outside diameter of the insulating tube.

It has just been found while calculating for the insulating tube that a secondary potential of 170 kilo-volts will be encountered. There must be therefore such a number of secondary turns as will, under normal working conditions of the primary, give this potential.

From 50,000 to 80,000 turns represents good practice for a 12-ineh coil. Simple calculations will show that at the primary tap giving 260 turns, which is a fair working point, and with an interrupter working on direct current giving an effective e. m. f. across the primary of 40 volts, the number of secondary turns for 170,000 volts should be 55,000.

The above calculations assume that with the given design of coil all the secondary is wound in the most effective part of the field and a large current is used such as would fully magnetize the core before "break." It is desirable, however, with the present design to obtain the spark with a comparatively small amount of current and it is preferable, therefore, to take advantage of data obtained from experience in the construction of other coils and use somewhere in the neighborhood of 70,000 turns.

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