Ans. A galvanometer and battery are connected in series with one terminal of the galvanometer connected to one set of brushes; the unconnected terminal of the battery is then connected with the other set of brushes. A deflection of the needle will indicate a short circuit. [Illustration: Fig. 698.--Field coil testing with telephone receiver. In the method here shown, a telephone receiver is connected in series with two symmetrically placed coils A and B. Very little sound will be heard when the flux through the two coils AB is the same; but if a short-circuited coil is being tested, the fluxes through the coils A, B will not be equal and a noise can be heard in the receiver.] Ques. What is the effect of a short circuit in the field coils or field circuit? Ans. The machine generally refuses to excite. Ques. How are the field coils tested for short circuit? Ans. By measuring the resistance of each coil with an ohmmeter or Wheatstone bridge. The faulty coils will show a much less resistance than the perfect coils. The fault may also be discovered and located by passing a strong current from a battery or another dynamo through each of the coils in turn, and observing the relative magnetic effects produced by each upon a bar of iron held in their vicinity. The short circuit may be in the terminals or connections, and these should first be examined and tested. Some series dynamos are provided with a resistance, arranged in parallel or shunt with the field coils, to divert a portion of the current therefrom, and thus regulate the output. When making a series dynamo excite, all resistances and controlling devices should be temporarily cut out of circuit by opening the shunt circuit. Series machines have frequently a switch which short circuits the field coils. Care should be taken that this is open, or otherwise the machine will not excite. [Illustration: Fig. 699.--Watson armature complete. The armature coils are form wound, heavily insulated and so mounted on the core as to insure rapid dissipation of heat by ventilation. Each coil is protected by an insulating sheath and tape covering before mounting. The armature is baked after the coils are mounted to drive out all moisture, then, while hot, is treated with insulating compound and again baked twelve hours.] Wrong Connections.--When a machine is first erected, the failure to build up may be due to incorrect connections. The whole of these latter should therefore be traced or followed out, and compared with the diagrams of dynamo connections given in figs. 190 to 198.