Page:Popular Science Monthly Volume 91.djvu/625

 Popular Science Monthly

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��student plan of learning will of course develop both the speed and reliability needed, a little at a time, but for the best results there should be some way to check and eliminate possible errors in sending which may pass by both students quite unobserved. The best way to take care of. this difficulty is by

the cooperation of a ^1*1*1

skilled "operator, as explained above. If no such help can be secured, the next best plan is to use an automatic sender of some sort. Such an auto- transmitter, driven by clock- work or by a small electric motor, can be used in self- instruction or by a group of students. Several types are available, but the two most employed use metal disks or paper tapes already pre- pared and carrying various combinations of words and letters in common usage.

The Disk Sender

The operation of the metal disk type is indicated in Fig. 9, where the disk itself is shown rotating in the direction of the arrow and having notches cut around its circum- ference to correspond with dots, dashes and spaces. As the disk re- volves, the point on the spring arm P moves back and forth along the irregu- lar teeth, and the contacts A and B are brought to- gether and separated to form dots and dashes. When the contacts are pressed together the bat- tery circuit from the dry cells D through the buzzer Z is closed by way of the pivot /, spring 5 and bind- ing post C, and the buzzer sounds. A short tooth on the disk produces a dot signal, and a long tooth a dash. Thus the notches shown in Fig. 9 would produce dot-dash, space, dash-dot-dot-dot, or the letters "A" and "B" separated by a space.

The metal signal-disks are made up in great variety, and are interchangeable so as to form complete message's. For beginning of practice there are disks giving the letters

���▼. " r Ba

��Diagram of connections for an automatic sender in a line

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9

PERIOD COMMA INTERROGATION PARENTHESIS QUOTATION DOUBLE DASH

FIG 12 The numerals and the most used punctuation marks in telegraphy

��of the alphabet singly and in regular order, and also in mixed order. This last named arrangement is especially desirable for practice, since it is almost impossible to anticipate the letters about to be sent, and the student is forced to listen closely and think quickly in writing the letters as they occur in what line wire _ amounts to a long

cipher word. Several forms of the disk sender are made, under the trade name of "Omni- graph," in which the disks are rotated either by hand or by clockwork. The automatically driven type , is by far the best for seri- ) ous work, since its speed may easily be adjusted to almost any rate which oc- curs in telegraphy, and, once started, the instru- ment may be put com- pletely out of mind and full attention given to writing out the letters and words pro- duced by the buzzer.

The Paper Tape Transmitter

Another sort of automatic sender is shown in Fig. 10, in which a cut paper tape passes between a spring contact and a metal roller. The sketch shows how the "slip" is threaded over a guide rol- ler G and between the two driving rollers E and F which turn in the direc- tions indicated by the ar- rows. The spring contact H bears down on the paper, and, when a hole in the tape comes under it, makes contact with the metal roller F. This closes the battery circuit from the cells B through the buzzer Z, and the buzzer reproduces the dots and dashes cut in the paper strip. On the left is shown the letter "R" (dot-dash-dot) and on the right the letters "A" (dot-dash) and "I" (dot-dot). An automatic transmitter of this general sort, in which the tapes are cut to give various word and letter combina- tions, is sold under the name of the "Audi- ble Alphabet." The instrument is driven

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