Page:Popular Science Monthly Volume 90.djvu/808

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��Popular Science Monthly

��number to fill a slot. As one side of a coil fills but half of the slot, 240 turns per coil will be the requirement. Now wind 14 such coils, all alike and in the same direc- tion. Use good wire. Never use a spliced or jointed piece of wire to make up a coil. Make a loop at the beginning of each coil before taking it off the form, so when the

���The winding and the commutator as they would appear if laid out on a straight plane

time comes to connect with the commutator, the inner and outer ends can be dis- tinguished. It is best to tie a turn of thread around the ends of the coils so they will not unwind or fall apart.

Now cut 14 pieces of Empire cloth and slip one in each slot, as in Fig. 4. Cut the cloth so it will project 3^ in. past each end; the extra cloth sticking out on top will come in useful later. Now place the coils. Since the bundle of wir# making up a coil is too thick to pass through the slot open- ing, the wires must be put in a few at a time until they are all in the slot. Squeeze them down to the bottom. Use no sharp tools to do the work. All the short sides of all the coils are successively put in place first. Always lay or place a given coil over and not under the one before. It is needless to say that the end of the coil with the terminals should face the commu- tator.

The placing of the coils is shown in Fig. 5. With all the coils half in, it is now well to connect them together. Start with any beginning wire of a coil and twist it to- gether with the end of the coil occupying an adjacent slot. Keep on going around the core in the same direction. You will see that every coil on the core is in series with the one next to it. This is where our drum winding appears similar to a Gramme winding. Now tie a string around the whole core and coils and tighten it. Put your finger on any slot, and count slots in the direction the other side of your coil

��goes till you come to number 8. The slot your finger was on at the start counts as number one, so the coil will occupy slots I and 8.

If the coil does not seem to reach, keep on working all the coils toward their slots until they come down to shape. When all the coils are in, cut the extra Empire cloth off on top till only about 3^ in. remains. Now, with the back edge of a knife, press these projecting edges of cloth in under the edges of the slot, as shown in Fig. 6. Then a fiber or thin wooden wedge can be driven in the top of the wire. After this, leave the whole armature in insulating varnish, enamel or shellac for a few hours, and bake the armature dry in a moderately heated oven.

When dry, scrape all enamel from the commutator and shaft. Now determine where the leads from the coils go to the commutator. If the brushes are placed midway between poles, have the lead come from a coil to such a bar that when the brush touches that bar the coil is out of the field, or in the gap between the field poles, as in Fig. 7. This is easy and all the rest of the leads follow successively. When you have them all soldered to the segments, the job is completed.

The winding just described is depicted in Fig. 8. This method of form winding has an advantage over hand winding, be- cause on small iio-volt motors there are so many turns per coil that hand winding would be exceedingly slow and tedious.

In the case of the commutator having twice as many segments as core-slots, bring out loops from the coils when half wound. These terminals are joined to the alternate bars. It is simply a scheme to have a larger number of bars, and thereby cut down the voltage across each bar. If you have a 12-slot core and 6-segment ^ commutator, you can make a coil occupy' a whole slot or else connect two coils in series before any lead goes to the com- mutator. — George Sturley.

��Calibrating the Receiver for a Wireless Set

A ROUGH, but very useful wavelength calibration of a receiving set may be made as follows:

Carefully tune the loose coupler to those stations using known wavelengths, being careful to select them so that the complete range of the receiver is covered, as 200, 300,

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