Fig. 99.
210. The two stations, R and L, may be near each other, or in different houses. The return wire, R W, passes from the copper of b to the zinc of B. This is important. If the cells are not joined properly, they will not work. It is better to have the cells together, on a short line, joined in series. The line wire, L W, and the return wire, R W, may be made of insulated copper wire for short lines in the house. Ordinary annunciator wire, No. 20, is good and cheap. The kind that is double cotton wrapped, waxed, and paraffined, has about 235 ft. to the pound. You should get at least 5 ft. for 1 cent. If your line stretches from one house to another you will find it better to use iron wire. Galvanized iron or steel wire No. 14 is good. This size weighs about 100 lbs. to the mile. The return and line wires must not touch each other at any point; they must not touch any pipe or other piece of metal that will short circuit your batteries. It is best to use porcelain or glass insulators to support your wires if the line is long; but for short lines, where you use a return wire, you may support the wires upon poles or trees by means of loops made of strong cord or wire.
211. Operation. Suppose R (right) and L (left) have a line. By studying Fig. 99 you will see that R's switch, E, is open while e is closed. The whole system, then, has but one place where the circuit is open. As soon as R presses his key, K, the circuit is closed, the current from both cells rushes around through K, S, L W, s, k, b, R W, and B. This magnetizes the bolts of both S and s, and their armatures come down with a click upon the regulating-screws, where they remain as long as the current passes. As soon as R raises his key the armatures rise, making the up-click. R can, in this way, regulate the time between the two clicks. If he presses K down and lets it up quickly, the two clicks that his friend L hears from s are close together; this makes what is called a dot. If R holds K down longer, it makes a longer time between the clicks for L to hear, and this makes a dash. R, of course, hears his own sounder, which is making the dots and dashes also.
As soon as R has finished, he closes his switch, E. L then opens his switch and proceeds to answer. Both E and e should be left closed when you are through talking.
(Read [§ 194], [195], and study what is said in [App. 9] about the gravity cell to be used on such a line.)
212. Telegraph Alphabet. The letters are represented by combinations of dots, dashes and spaces. A dot is made by pressing the key down, and raising it at once; that is, the key is raised as soon as it strikes. This makes the letter E. The dash is made by pressing down the key, and allowing the current to pass about as long as it takes to make 3 dots; this makes the letter T. A long dash for L should take about as long as for 5 dots. Spaces occur in a letter and between words. To make a dash you hesitate while the lever of the key is down, to make a space, you hesitate while the key is up. H is made with 4 dots without hesitation or space. By putting a space between the dots the letter &, Y or Z is made according to the position of the space. Notice that letters containing dashes do not contain spaces. A space is really the opposite of a dash. The letters C, E, H, I, O, P, R, S, Y, Z, and & are made entirely of dots or of dots and spaces.
You should notice that several letters are the reverse of others; A is the reverse of N, B of V, D of U, C of R, Q of X, and Z of &. The student should study some book upon telegraphy, if he desires to become expert. Punctuation marks are left out of the alphabet here given, as boys will find very little use for them.
