XVI.

CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF
THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY.

XLI.— ON THE EXCURSION OF THE DIAPHRAGM OF
A TELEPHONE RECEIVER.

BY CHARLES R. CROSS AND HENRY M. PHILLIPS.

Presented January 11, 1893.

THE present investigation is a continuation of one upon the same subject by Messrs. Cross and Mansfield, the results of which have already been published in these Proceedings.*

In the study of the excursion of the telephone diaphragm described in the paper referred to, the current employed was an alternating current from a dynamo-electric machine, the magnitude of which current was considerably above that of the telephone currents employed in practice. In the present investigation we have employed the currents furnished by a powerful long-distance microphone transmitter to actuate the receiver, and have measured the amplitude of the vibration of the diaphragm of the receiver under such currents when the strength of the polarizing magnet was varied.

The apparatus employed was substantially indentical with that described in the paper referred to. A more sensitive electro-dynamometer with a unifilar suspension was used, and the support holding the disk of the receiving telephone was made somewhat more rigid, as it was found that it yielded slightly under the strong pull of the polarizing magnet when this was strongly magnetized, and in close proximity to the diaphragm. The microphone, actuated by a C, stopped organ-pipe of large scale, blown by a carefully regulated blast, was placed in the primary circuit of an induction coil, such as is ordinarily employed with the long-distance transmitter. In the secondary circuit was placed the receiving telephone, which was to be studied, and also the electro-dynamometer and an adjustible water resistance. In a few experiments the ordinary hand magneto-receiver was used, but in most

* Ante, page 93.

cases we employed an experimental receiver like that used in the earlier studies referred to. The core was a bar of Norway iron, 3 inch in diameter and 8 inches long, wound with 2,750 turns of No. 18 (B. & S.) insulated copper wire. The current which traversed these coils was furnished by a dynamo machine. It passed through an incandescent lamp and a variable resistance frame, with which last the magnet was placed in derived circuit. By varying this current any desired strength of field could be obtained. In all our experiments the core of the electro-magnet was far removed from saturation, so that we may assume the strength of its field to have been proportional to the current traversing its coils. The strength of this current was measured

in the earlier experiments by a Thomson graded galvanometer, and in the later ones by a Weston milliamperemeter. The transmitter current was furnished by a single storage cell, and the organ-pipe which served as a source of sound was placed in a distant room. The arrangement of the stroboscopic fork was similar to that used in the previous work. The first measurements were made with an ordinary magneto-telephone as a receiver. The line current was varied, and the corresponding excursion of the diaphragm determined, with the results given in Table I. Series C is graphically illustrated in Figure 1. The values given in the table are the mean of three or more closely concordant readings. In this and the following tables the current is given in milliamperes, the excursion in terms of one micrometer division as a unit. In the figures the abscissas represent currents, and the ordinates excursions.

It appears from these results that with the magneto-receiver now used the change in the amplitude of the vibration is approximately proportional to the change in strength of the current for the stronger currents used in the measurements, but with the weaker currents the amplitude increases at a more rapid rate.

These currents as a whole,

however, are higher than those reached in practice.

In subsequent experiments we used as a receiver the electro-magnetic telephone previously mentioned. The end of its core was set at a distance of inch from the diaphragm. With the undulatory current in the line coil maintained at a constant value in each separate series of experiments, while the magnetizing current was varied, results were obtained as shown in Table II. Line currents with values successively varied in the different series were produced by the microphone transmitter. The values of the magnetizing currents in milliamperes are given in the first column, the corresponding excursions of the diaphragm in terms of one division of the micrometer in the second. Series I., III., V., and VII. are represented by the curves in Figure 2.

The results reached agree very well with those obtained by Cross and Mansfield, when currents from an alternating dynamo were used, except that in these last the maximum value of the excursion was reached with a magnetizing current of 320 milliamperes, while with those obtained in the present investigation the maximum value was

reached when the magnetizing current was only 225 milliamperes. This difference arises from the fact that in the earlier experiments the end of the core of the magnet was set at a distance of inch from the diaphragm, while in the present ones it was at a distance of only inch, and so approached saturation sooner.

Experiments were also made with the same apparatus in which the strength of the field magnet of the telephone was kept constant by maintaining a constant current in its coils, while the line current was varied. In the first series of these the diaphragm was separated from the end of the core by a distance of inch. The results are given in Table III., and Series VIII., IX., XI., XIII., and XIV. are graphically illustrated by the curves of Figure 3.