(The following is reproduced from Dyke's Automobile and Gasoline Encyclopedias, 7th Edition, 1918, p.304 and has been retyped and edited by Ken Lauderback of Milwaukie, OR)
No matter what grade of material is used, if the design and workmanship put into building an electromagnet for recharging magnetos is not correct the completed job is very liable to be a failure. The design and construction of a very simple charger suitable for operation from a 110-volt direct current lighting circuit is given herewith.
If alternating current is used a rectifier must be used between the lighting circuit and the coils.
Sizes of Iron Cores: In Fig. 1 is shown a piece of cylindrical soft iron, 1 inch in diameter and 6 inches long. This piece constitutes one of the cores. Cores should be made in one piece, because there is less loss in magnetism than in multiple-piece cores. The dimensions above given have been shown in practice to give the best results. A short and comparatively thick electromagnet is more efficient than a thin one.
Paper should be wrapped about the cores, this being put on in layers and over each layer a coating of shellac applied. The paper insulation should be laid on until a thickness of about 1/8 inch has been built up.
After the cores have been properly insulated, the next step is to wind them. This particular design calls for about 14 pounds of 18 B&S gauge, cotton covered copper wire. [Editor's note: cotton covered wire is not available, use regular 18 gauge magnet wire. KL] This means that about 7 pounds or 1,800 feet should be wound around each core.
With this amount of wire each coil will have about 3,100 turns. This, of course, makes the electromagnet one of about 6,200 turns. The coils consume 3 amperes. The number of ampere turns equals the number of turns times the number of amperes, or 18,600 ampere turns.
A very important thing which must be remembered in winding the wire around the cores is that the windings should be in opposite directions, as shown in the sketch. The wire should be started with about 2 feet left over, so that the connections with the circuit may be easily made. In winding, each turn should be flush against the previous turn and tightly drawn. It is well to look to the insulation of the copper wire frequently as the winding progresses.
After both cores have been wound, they should be connected as shown in the sketch. One end of the core is attached to one of the other core. This leaves two ends to be used for connecting to the lighting circuit. A base plate is required for mounting the cores.
A base plate of iron is best suited and this should be of 1/2 inch stock, big enough to hold both windings. Two holes should be countersunk into the base plate, the diameter of these holes being equal to the diameter of the windings. These holes should be spaced about 1/2 inch apart. The cores of the windings must surely touch the base plates. To be sure that they do, they should be pounded in.
The complete cost of the apparatus should be under $25  when completed. Some means of control, must of course be had, and a switch where the current is taken from the lighting circuit is best suited.
To Charge: The magnet is placed on the top of the cores, north pole of the magnet to be charged being placed on the south pole of the core of electromagnet and south pole of magnet, on north pole of electromagnet.
The switch contact is closed and opened three times; for periods of 5 seconds each and vibration should then be set up in the magnet by lightly tapping on each side near the top. This can be done quickly and properly by using two 6 oz. Hammers.
The time required to fully recharge a magnet varies from 30 seconds to 1 minute. Open the switch and take the magnet off and immediately put it back on the magneto or place a bar of soft iron across the ends, called a "keeper."
Magnets Made of Steel: The magnets used on both low and high tension machines are of special tungsten steel made as hard as it is possible to obtain them, so hard that a sharp file cannot make any impression on the metal. Much depends on the class of steel used--a special grade known as magnet steel is now being adopted. The retention of magnetism by steel is a very curious and interesting property. It resides only on the surface of the steel, and it is found that a much stronger magnet is obtained, weight for weight, by making it in sections, one placed over the other, than by using a massive single magnet. Magnetos have two magnets placed side by side; some have a single large magnet--they were formerly two--superimposed.
Soft steel is easier to magnetize than hard steel, but the former loses it quickly if submitted to vibration. The hard steel magnet loses its magnetism very slowly, although the magneto has, as a matter of course, to withstand much vibration from the engine, etc. Any standard type of magneto should show very little loss of strength for two years, but after this period it is as well (though not an actual necessity) to have the magnets brought up to full strength again.
Notes: I built one of these chargers for a friend about 15 years ago. I would make the following suggestions: Use thin wall tube of plastic or nylon to insulate the central core, rather than paper. I tried electrical tape; the wires cut through it. The magnet wire, although wound on a lathe, had a tendency to want to unwind. I would suggest coating the coils with shellac, epoxy, or resin, to help hold them together. The rectifier I used is a small device from an electronics store. The charger worked very well. Magnet wire is available from EIS, Inc. in Portland, Or. Phone (503) 227-3601.
Good luck, Ken Lauderback Milwaukie, OR (503) 653-9459