792 



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, 



