Fig. 157
The boys unscrewed the end of a telephone receiver ([Fig. 157]) and found inside a permanent magnet made of several steel bars bolted together ([Fig. 158]). This was shown to be a magnet by presenting a small pocket compass to either end. The left-hand end of this magnet proved to be its north pole by repelling the blue end of the compass needle.
Fig. 158
Fig. 159
On the left-hand end of the magnet was a small spool of No. 36 copper wire, silk covered. It offered 75 ohms of resistance, and since it takes 2½ feet of this wire to furnish 1 ohm of resistance the spool contains 187½ feet. A thin disc of soft iron .01 inch in thickness is held by the hard rubber case very near to but not quite touching this end of the magnet. We drew this disc to one side, as shown in [Fig. 159], and connected the receiver by wires to a magneto. We turned the crank of the magneto slowly and the iron disk danced up and down, keeping time with the revolutions of the armature. The magneto furnished an alternating current, which, when it flowed around the coil in one direction, strengthened the pole of the magnet, and in the reverse direction weakened the pole. When the crank was turned so as to produce twenty to thirty revolutions of the armature per second the dancing of the disc sounded like the low hum produced by the wing of a humming bird. When a large, wide-mouthed bottle was brought near to this the sound was greatly reinforced, as the sound of a bee becomes louder when he appears at your open window. We next replaced the iron disc and put on the cap again. We then connected the receiver at S ([Fig. 160]) and connected two dry cells at p. When the primary circuit was closed the disc vibrated in time with the hammer of the bell making the same tone. We substituted for the bell a series of buzzers. The smallest had an armature about one inch long, while that of the largest was about two inches long. The shorter the armature the faster it vibrated, and the higher was the pitch of its tone. We arranged these as shown in [Fig. 161]. A, C, D, E and F are the buzzers. B is a battery of two cells and G, H, I, J and K are springs of sheet brass which act as push buttons. By operating upon these springs with one's fingers, as upon the keys of an organ, it was possible to represent the tones of a reed organ after a fashion. The armatures are reeds and they are made to vibrate by electro-magnets. We called it an electric organ. The telephone receiver was connected at T, and the wires which led to it were lengthened so that the receiver might be a long distance away. The disc in the receiver kept time with the armature of each buzzer when it sounded and faithfully reproduced its sound. But the strangest thing was that when any two buzzers sounded together, or, indeed, if all five buzzers sounded together, the receiver responded to them all at the same time, so that a person in another room or in another house, with the receiver at his ear, might hear exactly what those did who were in the same room with the buzzers. The wires from the receiver were connected with the coil in each buzzer so as to get the induced current, as shown in detail in [Fig. 160].