In each of the spool heads turn out a round recess 1/4 of an inch deep and of such a diameter that the paper tube 7 will just push tightly into the recess. Now put some thick shellac varnish on the tube ends and push the two heads on, see that they are square with the tube and allow the shellac to dry over night.

Procure about 25 pounds of number 16 B. & S. gauge single cotton covered magnet wire. Mount your spool on a square bar of wod in the lathe and arrange back gear so that the spool will turn very slowly. Wind a bunch of tape on the wooden ber and. up against the outside of each spool head so that the heads will not be pushed off of the tube as the wire is wound on.

Drill a hole in one of the heads close to the tube and pass the end of your wire through for about 6 Inches. Cover the tube with one layer of the Grimshaw tape and wind on a layer of the wire allowing it to pass under a round brass bar placed close to the spool so that by holding the wire with the hand at the proper angle, it will wind on under great tension and consequently very tight. With care you should get about 90 turns of wire on each layer. Give each layer a good coat of thin shellac varnish as soon as wound and when this is good and hard put on a turn of cotton or linen cloth, fastening it at the edge with flour paste and seeing that it is pulled tightly in

Fig . 2. Complete place. Now wind another layer of wire and so on until you have wound just exactly 600 turns. Now bare your wire for a 1/2-inch and solder on a piece of thin brass or copper about 1/4 inch wide and about 2 1/2 inches long and bent to receive the wire as shown in Fig. 3. Take a piece of the tape about 2 inches long and place it under where this tap lies down on the coil, so that when the sides of the piece of tape are bent upwards and over they will cover the soldered joint and also the wire for about an inch on each side of it. Wind on another 600 turns and when you are pasing the first tap be sure and put a piece of tape on with its end under the turn of wire nearest to the piece of brass or copper. Do this on each side of the tap and leave the tape long enough so that each succesive layer of wire will have tape between the piece of brass and the turns of wire on each side of it. Put another tap on the end of the second 600 turns and wind on another 600, not forgeting to shellac each layer of the coil and to put the piece of cloth between the layers the same as you did at first. Solder a tap at the end of each 600 layers until you come to the fourth tap. You should now have 4 taps all in a row and in order that you may make no mistake you can now put a label on each tap and on the inner end of the wire that you passed through the hole in the spool head. Mark the inner end 400, the first tap 300, the second tap 200, the third tap 100, and the fourth tap 0. Now wind on 50 turns and solder on a tap marking it 5, another 50 turns and mark tap 10, and so on marking the tap at the end of each 50 turns 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 90, 95 and 100. The taps from 0 to 100 should be placed in such a position that they will occupy another line a little separated from the line occupied by the first 4 taps. Finally wind a layer of tape over the outer layer of wire to keep everything in place. You will now find that if you have wound tightly and the pieces of cloth used between the layers have been no thicker than a thin handkerchief, that your spool will be just about full and it should be let stand for a few day to thoroughly dry in a dry place.

Remove the 2 bolts in leg 42 of the core, open the joint and slip the spool in place, close the joints and replace the bolts. You will find some considerable difficulty in putting the joints together and it will help you wonderfully to cut a number of pieces of thin iron or tin about 1 1/2 inches square and bend them to the shape shown in Fig. 4. Place one of these over each long section of the ends of the core and then you will have but little difficulty in putting the joints together and you can remove the pieces of tin with a pair of pliers after the joints are well entered into place.

We should have told you that in drilling the hole in the spool head for the inner end of the wire it should be drilled over the center of one of the sides of the square hole as if it is drilled over one of the corners of the square hole, it will come dangerously near to the iron core when the spool is put in place. Then another thing that you must be ecareful about is to so place the spool on the core that this same inside end of the wire will be as far away from the core as possible.

Your transformer is now complete and you can either mount it on a board or in a box and solder rubber covered No. 16 wires to the different taps and lead them to properly numbered binding posts either on the outside of the box or on the base board.

If your circuit is 52 volts or thereabouts you can now connect the circuit to numbers 0 and 55 or to any other two numbers that have 55 turn between them, or even 50 turns will not be too few. If your cicuit is 104 volts or thereabouts you should connect to numbers 100 or 10 or any other two numbers that have 100 or 110 turns between them.

After the transformer has been duly connected as above to either a switch or key socket you can turn on the current and if you have been careful with your work only a very small amount of current will flow through the coil and if you have a 5 ampere fuse in circuit it will not blow. If, however, you have failed to properly insulate the pieces of brass or copper from the adjacent turns of wire with carefully placed tape, or if you have been careless with your work in one of many other ways, you will perhaps have a short circuit and even a 25-ampere fuse will be instantly blown and you will have to unwind your coil and find the trouble.

Assuming that everything is all right you can connect a wire to number 0 and another to number 5, this, will give you 5 volts for your work, 0 and 10 will give you 10 volts, 0 and 15, 15 volts, and so on until you get to 0 and 100 which will give you 100 volts. To go above this remove the one wire from 0 to 0100. This will mean that you must add 100 to the number to which the other wire is connected so that if it is at 15 you will have 115 volts and if it is at 95 you will have 195 volts. To go above 200 you must remove the-one wire from 0100 to 200 and this will mean that if the other wire is at 35 you will have 235 volts. To go above 300 remove wire from 200 to 300 and to go above 400 remove it from 300 to 400 which will give you up to the limit of the transformer at about 500 volts.

You must be very careful not to allow bare wires to touch and thus short circuit a portion of the transformer and either overheat it and perhaps burn it out or blow the fuse. Then again don't forget that a voltage above 200 is dangerous and sometimes fatal so you must use the utmost care in handling your transformer and it is wise either to keep one hand behind your back or else use good rubber gloves.

One of the purposes that this transformer is very convenient for is to heat the wire used in the building of induction coils. When the sections of your coil are all stacked up and the connections made, whether the wire has been wound in wax or not, you should pass a current through the wire sufficient to warm it and thus drive out all moisture, and if the coils have been wound in wax this treatment will soften the wax and allow the sections to settle down upon each other and after cooling it will be found that the whole is one solid mass. Be sure, however, that you do not overheat for you must remember that the heat inside of the coil is much more than that which you feel on the outside and it is easy to char the insulation and thus-ruin the coil. The way to do is to try a small voltage for a few moments and then gradually increase until you get enough voltage to drive sufficient current through to warm the coil. It may happen in very large coils that 500 volts will not be enough and in such a case warm up half of the coil at a time.

While this transformer is designed for a frequency of 60 cycles it will work equally well on a higher frequency.

"The American Inventor."