Page:Book of record of the time capsule of cupaloy (New York World's fair, 1939).djvu/48

 should be connected an amplifier which in turn is connected to some type of current indicator, such as a galvanometer or telephone receiver]. Some means should be provided for accurately measuring the strike or direction of the coil in the horizontal plane, as well as its dip or deviation from the vertical position. On level ground, where there is nothing to distort the primary field, the current generated in the small, or pickup coil will be at a minimum [that is, produce the least deflection of the galvanometer needle or the least sound in the telephone receiver] when its plane is perpendicular to that of the large coil. Conversely, the maximum current will be observed when the two coils are in the same plane. It is well to take both observations as a checkup before beginning the search for the Capsule. If the instrument is working properly, the positions of minimum and maximum current in the pickup coil should be at right angles to each other.

In exploring for the Capsule, observations may be made with the pickup coil in two ways.

First: Take measurements in the plane of the energizing loop, moving farther and farther away from it in short stages of five or ten feet. Do not work too close to the energizing loop. If during this survey the pickup coil passes over buried metallic material it will be noted that the positions of the coil do not correspond to those described for an undistorted field. The divergence from the normal dip will be at a maximum over the hidden body, whereas the deviation from the normal strike will increase as the metallic substance is approached, reverse to a maximum in the opposite direction as the spot is passed