472 



ropuiar Science Alonihly 



Making Small Generators from 

 Telephone Magnetos 



THE amateur electrician may con- 

 struct very sturdy little direct cur- 

 rent generators of either series, shunt or 

 compound type from the parts available 

 in an old telephone magneto which may be 

 procured for a few cents from nearly any 

 telephone exchange, especially in the rural 

 districts. It does not matter whether the 

 magneto is in a workable condition or not, 

 providing the parts are all intact. 



The first operation must be that of 

 properly reconstructing the 

 armature in order to make it 

 adaptable for the generation 

 of direct current in place of 

 the alternating current, 

 which telephone magnetos 

 produce. First re- 

 move the fine wire 

 which is wound up- 

 on the armature 

 and replace it by 

 ■winding each pole 

 of the armature full 

 of No. 22 single cot- 

 ton covered wire. 

 It is very necessary 

 that both poles be 

 wound in the same 

 direction as shown 

 course, understood 



A finished generator 

 from parts of an old 



in Fig. 1. It is of 

 by amateur elec- 

 tricians that the smaller the wire con- 

 tained on the armature of the dynamo 

 the higher the voltage generated will be, 

 with a corresponding decrease in the am- 

 perage. Telephone magnetos are de- 

 signed to generate a potential of several 

 hundred volts in order to overcome the 

 high resistance of the line. This high 

 voltage and low current value is not 

 suitable for practical purposes, especially 

 in the amateur's workshop. Hence, 

 the necessity of changing the small wire on 

 the armature to that of a larger size. 



As the armature is a two-pole affair, the 

 commutator will need but two segments. 

 The commutator will be of the disk type as 

 it is very simple to make and possesses 

 certain advantages over the drum type 

 which make it more adaptable for this 

 purpose. Owing to the hollow shaft of 

 the armature, which is used to bring the 

 leads to the cummutator, it will be found 

 rather difficult to fit a drum commutator 



to it. The details of the small disk com- 

 mutator are shown in Fig. 2. The copper 

 segments are fastened to the fiber disks by 

 means of small brass brads used as rivets. 

 The brads should be filed perfectly flush 

 with the surface of the copper segments 

 after being hammered into place. The 

 hole in the center of the fiber disks should 

 be a trifle smaller than the shaft so it can 

 be forced on the shaft and made to remain 

 rigidly in olace. After the two leads from 

 the winding are brought through the 

 hollow shaft and soldered to the segments, 

 the armature of the machine is completed. 

 The construction of the armature 

 is just the same whether the gene- 

 rator is of the shunt, series or com- 

 pound type. 



Only one magnet of the magneto 

 is used in the construction of these 

 machines. If a more powerful 

 generator be desired, two magnets 

 may be used. In 

 fact, the three mag- 

 nets may be used 

 together without 

 any winding at all 

 by merely taking 

 the current off the 

 commutator with 

 two small brushes. 

 The writer would 

 advise, however, that either one or two 

 magnets be used with a field winding. 

 Aside from giving the mechanic a better 

 understanding in dynamo construction, 

 the types with field winding possess cer- 

 tain advantages over those without it. If 

 the mechanic wishes a series wound ma- 

 chine, the field coils should be wound with 

 No. 24 single cotton covered copper wire. 

 A little more than ]i lb. will be needed. 

 Paper is first wound around the magnet to 

 insulate it from the wire. It is not neces- 

 sary to wind the wire on carefully or to 

 make bobbin heads to hold it in place. It 

 is essential, however, to have approxi- 

 mately the same amount of wire on each 

 pole. It is also necessary to wind each 

 field coil in the same direction. After the 

 field coils are wound, they are given a coat 

 of shellac and covered with friction tape, 

 leaving the leads protruding for con- 

 nections. It is also desirable to shellac 

 the tape after it is wound on to make the 

 winding as nearly moisture proof as 

 possible. 



which can be made 

 telephone magneto 



