790 



storage battery. As most charging gen- 

 erators are wound with a very low re- 

 sistance in the armature this would of 

 course put a short circuit across the battery 

 and ruin it in a very short time. In some 



Popular Science Monthly 



A low resistance electromagnet in series with 

 a charging circuit to prevent a short circuit 



cases it would reverse the polarity of the 

 dynamo. A very good way to prevent 

 such occurrence is to insert a low resistance 

 electromagnet in series with the charging 

 circuit that will attract an armature which 

 forms a part of the charging circuit, as 

 shown in Fig. I. 



As seen in this diagram the armature, 

 when against its front stop, forms the 

 circuit from dynamo to the batteries. The 

 magnet will hold the armature in contact 

 as long as the voltage of the dynamo 

 exceeds that of the battery, but as soon as 

 the voltage becomes equal, the magnet be- 

 comes de-magnetized and the battery cir- 

 cuit is consequently broken by the armature 

 being withdrawn with a spring against 

 its back stop, which makes a second 



FIG. 2 

 Here we have one coil of low resistance 

 wound with No. 12 wire in series and another 

 coil wound with many turns of fine wire 



contact with a circuit having a local 

 battery with a bell for giving the alarm. 

 This arrangement, however, has one draw- 

 back in that its armature has then to be 

 re-set by hand to complete the circuit 

 when the dynamo is again started. In 

 Fig. 2 is shown another diagram whereby 



this function is performed automatically. 

 It will be seen that another coil has been 

 added. Here we have one coil of low 

 resistance wound with No. 12 wire in 

 series, and another coil wound with many 

 turns of fine wire, No. 34 or 36, but con- 

 nected in shunt. This shunt coil takes 

 very little current and serves to draw the 

 armature down to its front contact and 

 close the battery circuit as soon as the 

 generator has acquired sufficient speed to 

 energize it. 



These diagrams are drawn simply to 

 show the theory, but the finished article 

 adapts itself admirably to the ordinary 

 Morse telegraph relay, Iwith only one 

 change which will probably have to be 

 made in relay. Procure a Morse telegraph 

 relay from any electrical house. These 



A telegraph relay having both coils rewound 

 and connected in the line for actual use 



are generally wound for 35 or 150 ohms. 

 Rewind both coils, one with as much 

 No. 34 or 36 wire as it will hold neatly, 

 and the other with No. 12; or, if your 

 charging current exceeds 30 amperes, wind 

 it with No. 10 wire to avoid undue heating 

 which would char the insulation. As 

 most relays have a small piece of No. 30 

 wire, which forms a flexible connection 

 between the armature and its bearing 

 to preserve the contact, this will have to 

 be removed, as it will not be able to carry 

 the large current for which it will now 

 be used. The plan is to remove this 

 small piece of wire and insert a piece of 

 No. 14 wire in such a manner as not to 

 give the armature too stiff a movement. 

 However, this may not be necessary if 

 the trunnions of the armature fit snugly 

 in their bearings and preserve a good 

 contact at all times. When you have 

 both coils wound make the connections as 

 in Fig. 3, which shows the apparatus in 

 service. You may be able by the use of 

 these diagrams to construct this apparatus 

 without a Morse relay. — W. A. Poling. 



