Part 25

Another key might bring into action a dynamo which gives 64 vibrations per second to the diaphragm of the telephone receiver. This would send forth a tone very nearly like the base note of our 60-cycle alternating current dynamo.

The following table shows a series of ten tones which might be produced by the same little piece of sheet iron in a telephone receiver played upon by ten dynamos at the same time. The whole list of ten tones would sound well when produced simultaneously. The great mystery is that the iron disc can vibrate in such a complex manner. It is important to note, however, that the number of vibrations in each of the upper tones is a multiple of that of the lowest tone:

2nd octave above C´´--1024 (= 16 × 64) Middle C G´ -- 768 (= 12 × 64) E´ -- 640 (= 10 × 64)

1st octave above C´ -- 512 (= 8 × 64) Middle C G -- 384 (= 6 × 64) E -- 320 (= 5 × 64)

Middle C C -- 256 (= 4 × 64)

1st octave below G -- 196 (= 3 × 64) Middle C C, -- 128 (= 2 × 64)

2nd octave below C,,-- 64 (= 1 × 64) Middle C

The tone most easily reproduced by the vocal cords of a man. The tone most easily reproduced by the vocal cords of a woman. The tone which the telephone receiver responds to most readily. The table covers the range of the human voice, male and female.

All the intermediate tones, with their sharps and their flats, are produced each by its own separate dynamo.

The insignificant amount of current required to operate a telephone receiver makes it possible to furnish the music of these dynamos to many and far distant telephones. This naturally suggests the idea of having a great musician perform upon the keyboard and have many auditors scattered about the city in their private homes or even in many public halls, for the telephone receiver can readily be made audible to a good-sized audience.

XVIII

ELECTRIC BELL OUTFIT FOR THE COTTAGE

The boys asked me what arrangement of electric bells we needed at the cottage and so I gave them this problem to work out by themselves:

1. We want a bell in the kitchen to be rung by a push button at the front door. But there are times when no one is in the kitchen and hence,

2. We want a bell upstairs to make a single stroke whenever the kitchen bell is rung from the front door.

3. We want a floor push under the dining-room table which will cause the kitchen bell to ring a single stroke.

4. We want a push button in the dining-room which will cause both bells to clatter and call people from their beds, from the piazza, the lawn, etc., to their meals.

This equipment needs only one battery of two dry cells, two bells, three push buttons and about two hundred feet of wire. It should cost less than five dollars.

The boys drew many plans and tried many schemes and at last determined upon the plan shown in Fig. 173.

P is the floor push under the dining-room table. When the circuit is closed at this point the current leaves the battery from the carbon pole c, passes up and around the magnets of the kitchen bell and back to the zinc pole of the battery z by way of the push button P. All other circuits are open.

P´ is the push button at the front door. When the circuit is closed at this point the current leaves the battery at c, passes up to the right-hand binding post of the kitchen bell and divides, part going through each bell. The portion of the current which goes through the kitchen bell passes around the magnets and through the armature to the left-hand binding post before it can find a path back to the battery. Hence, the kitchen bell clatters. The portion of the current which passes to the upper bell goes around its magnets and finds a path back from the middle binding post to the battery by way of P´. Hence the bell upstairs rings with a single stroke.

P´´ is a push button situated upon the wall by the side of the door which leads from the dining-room to the kitchen. When the circuit is closed at this point, the current leaves the battery at c, passes up to the right-hand binding post of the kitchen bell and divides, part of it going through each bell. The portion which goes through the kitchen bell passes around its magnets and through its armature to the left-hand binding post, then up to the middle binding post of the upper bell, through its armature to its left-hand binding post and back to the battery by way of the push button P´´. The other portion of the current passes directly up to the right-hand binding post of the upper bell, around its magnets, and through its armature to its left-hand binding post, thence back to the battery by way of the push button P´´. Hence, both bells clatter and keep time with each other. The upper bell will ring independently of the lower bell, but the lower bell is dependent upon the upper one to open and close its circuit, somewhat as a relay.

Soon after the cottage had been equipped with electric bells I went to the mill one day and found a push button at the door. Upon going in I was curious to examine the electric bell outfit of that place and found what is illustrated in Fig. 174.

A switch, S, had been attached to the bell. The boys said that when they felt well they kept the switch upon the left-hand point and the bell rang as a clatter bell. When they felt a little sick they put the switch upon the middle point and the bell rang with a single stroke, but when they felt very sick they put the switch upon the dead point and the bell did not ring at all.

XIX

USING ELECTRICITY TO AID THE MEMORY

For the sparking equipment of the motor boat we use dry cells which have an internal resistance of not more than .06 ohm. They will, when short circuited through the ammeter for only an instant, give 25 amperes.

(1.5 volt)/(.06 ohm) = 25 amperes