This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
Prof. W. G. Clarke
At the present day nearly every experimenter has the alternating electric current in h s laboratory or else has easy access to a reasonable amount of supply. The voltage is usually either 104 or 52, and the frequency is usually 60 cycles.
It frequently happens that in your work you will need voltages both away below and away above this fixed amount. You can easily get a lower voltage by inserting' an adjustable rheostat in the circuit in series with your work. With this, however, you cannot get a higher voltage than the source of supply, and unless you have a voltmeter you can only guess at the amount of voltage that you are actually using.
By the use of an auto transformer you can get a range of voltage from 5 to 500 in steps of 5 volts and you know very close to the amount of voltage that you are using.
An auto transformer is not difficult to construct and the possessor of one will find no end of comfort and convenience in its use.
First procure a sheet of electrical steel 24x96 inches and .014 thick. Go to at tin shop and with the foot power squaring shears have them cut across the sheet, 8 pieces each 7 1/8 inches long, and another 8 pieces each 3 5/8 inches long. Then cut across these 16 pieces making a large number of pieces each 1 1/8 inches wide, and you will find that you will have 168 pieces 1 1/8 by 7 1/8, and 168 pieces 1 1/8 by 3 5/8. This is somewhat more than you need but it is well to have a few over as some are almost sure to be bent out of shape so that they cannot be properly straightened without a great deal of trouble.
The greatest care must be taken in setting the guages on the shears so that all the pieces will be exactly the same width at each end and also that they are cut perfectly square. Otherwise your transformer core will look anything but symmetrical although it will perhaps do the work just as well.
If you cannot get the electrical steel in your town we can tell you where to get it and also where to get it cut up ready for assembling in case you cannot find a squaring shears handy, but this last is unlikely as nearly every small tinshop has to have a foot power squaring shears.
Sheet tin or any kind of very soft iron will answer the purpose only that you will require a much larger number of pieces in order to get the required thickness of core.
Make up 16 lots of each length of pieces, each lot to have an equal number of pieces, and this number to be such that when 8 of the lots are clamped to/ gether tightly in a vise the whole will measure just 1 1/8 inches in thickness.
Build up your core by laying the several lots of pieces down on a board in the position shown in Fig. 1, and with the corners interlocked as the diagram clearly shows.
Now procure about a dozen small malleable iron clamps and clamp the whole core together tightly and evenly, tapping the corner edges with a hammer until all is square and symmetrical. If you have done your cutting and have used due care in assembling the pieces you should now have a very presentable core indeed, in fact some of these cores made up by amateurs are fully as god in every way as the cores made in the large factories with expensive automatic machinery.
Take your core on the board to a machine shop where there is either a drill press or a good lathe and drill 8 holes through the core with a 3/16 inch twist drill at the points shown in the diagram. A piece of hard wood should be placed under the core at the point where the drilling is being done so that it will receive the point of the drill when it comes through and thus prevent the formation of much "burr" at this point. A clamp should also be on each side of drill and as close to the hole as possible, the clamps can of course be moved around, one at a time, as the drilling progresses, care being taken however, to at all times have enough clamps on the core so that the pieces are tightly clamped all around.
Cut off 8 pieces of brass rod 1 3/4 inches long and No. 8 screw size in diameter. On each end of these pieces cut a No. 8/32 thread for a distance of about half an inch. Now procure 16 hexagon brass nuts No. 8/32, 16 brass or copper washers that will just go over your bolts, and 16 fiber or paper washers that will also just go over the bolts, but about 1/4 inch diameter. You can cut these paper washers out of cardboard.
Cut some strips of typewriter paper 1 1/8 inch wide and paste one turn of this around each bolt so as to insulate the bolt from the iron core. Now place the bolts carefully in the holes in the core, put a paper washer on each end, then a metal washer, and then the nuts. After all the bolts are in place you can screw up carefully with small bicycle wrenches until the core is tight all over and then you can remove the clamps. You will see the position of the washers and nuts by referring to 1, 2 and ) in Fig. 1.
Make a wooden roller carefully turned up to 1 3/4 inch diameter throughout its entire length of 8 inches. Cut a number of strips of cardboard 5 1/2 inches wide and the whole length of the sheet. Soak the cardboard in water and carefully remove one piece at a time and wind it on the wooden roller fastening it together with ordinary flour paste. Be careful, however, not to get any of the paste between the cardboard and the roller. Dry the tube out thoroughly either in the air or in a slow oven and then drive out the roller and you should have a very solid paper tube 1 3/4 inside and 1/1G of an inch thick. Trim off the ends with a sharp knife so that they are square with the sides of the tube and so that the tube will be not less than 5 3/8 and not more than 5 1/2 inches long.
Make two spool heads of perfectly dry hard wood as shown at 5 and 6, Fig. 2. Cut a square hole in the center of each of these so that they will fit tightly over side 42 of the core after it has been covered tightly with a layer of Grimshaw tape, which, by the way, should be put on this leg before the clamps are removed, the clamps can be removed one at a time as the taping progresses. The other three legs of the core are of course left untaped.