Abstract :
This paper describes an apparatus which has been developed for observing, by a direct deflection method, small changes in the effective resistance and inductance of a coil. It consists of a bridge network, with two inductive arms, which is normally balanced. The bridge is supplied with alternating currents, the indicating instrument being a d.c. galvanometer connected through a synchronously driven commutator. One of the arms of the bridge contains the ¿exploring¿ coil, which is placed on the metal surface to be examined. It is shown that if the commutator is so adjusted as to indicate the current in phase with the applied voltage, its deflection is proportional to the change in effective resistance of the exploring coil. With the commutator adjusted to observe currents 90° out of phase with the applied voltage, the deflection depends on the change in effective inductance of the coil, when this change is brought about by the presence of a metal plate. For non-magnetic materials it is best to use the commutator in the ¿out of phase¿ position. The change in effective inductance depends on the specific conductivity of the plate, and the apparatus may be used for quickly comparing the specific conductivities of metal plates. If such plates have cracks normal to the surface, their presence is indicated by a reduction in the galvanometer deflection as the coil passes over a crack. For magnetic materials the commutator should be set in the ¿in phase¿ position. Cracks in magnetic materials can be observed in the same way as those in nonmagnetic materials.
