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equal, the magnet be- comes de-magnetized and the battery cir- cuit is consequently broken by the armature being withdrawn with a spring against its back stop, which makes a second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.
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