Page:Electricity (1912) Kapp.djvu/199

Rh containing 100 wires would have a 50 part commutator, and every second wire would be connected to this commutator. Instead of single wires, groups of wires made up into coils may be used. It will be obvious that the e.m.f. generated in the armature will be the greater the greater the number of wires onthedrum. If thena high e.m.f. is required, the number of wires, counted all round the armature, may easily become so great, that if we have to provide a segment in the com- mutator for every second wire, the commuta- tor would have to be made up of exceedingly thin segments. For obvious reasons there is a limit to the decrease in the thickness of commutator segments, so that the rule, that one segment must be provided for every two wires, cannot always be adhered to. This difficulty may be overcome by grouping the wires in coils. The number of segments need then only be as large as the number of coils, that is one half the number of coil sides. Each coil side then takes the place of one individual wire.

There is in connection with drum winding a purely mechanical difficulty which must be mentioned. I said above that the connection between the front end of one wire to the front