This set can be made in an hour from scrap sheet iron and will work up to one hundred feet. The wire for the magnet can be taken from an old bell or bought for a few cents. The whole cost will not exceed twenty cents. If you must buy the wire, get No. 22 single cotton-covered magnet wire; about two ounces will do.
The armature (A) is a piece of sheet iron three inches long and three-eighths of an inch wide. About one inch from the end, solder a piece of needle (N) or hairpin three-quarters of an inch long.
The support or bearings (R) is a piece of sheet iron three and a half inches long, three-eighths inch wide. One and three eighths inches from each end, bend up at right angles as in sketch. This will leave exactly three-quarters of an inch between the top ends. Exactly in the centre of the bottom punch a hole with a sharp nail to let the screw through to hold it to base. Stand up and measure one and three-sixteenths from bottom on inside and make a dent with nail deep enough to clasp needle (N) on each end. For the base, use a block of wood three inches long, two inches wide, and half an inch thick.
Half an inch from one end, bore a small hole to take a screw (C) one and a half inches (if you drive the screw in without boring a hole it might split the block). Slip two little washers cut from cardboard three-quarters of an inch diameter on the screw. These are only to hold the wire from slipping off. Now drive the screw down until there is just one and one-eighth inches from the top to the block. Three-quarters of an inch from the other end of the block bore a hole to take the screw to hold the bearings down. One-quarter of an inch from same end drive a thin nail (F); leave just one and one-eighth inches from top to block. In the end of the block drive a small tack (T).
Wind the wire on the screw (C) the same as the thread on a spool. When you wind all the way to the bottom keep turning the wire in same direction till top is reached, then go down again. Wind five layers on and tie the end of the wire with thread so it won’t slip. Now put it together as in the sketch. The needle will fit tightly in the dents in the bearings so the armature can seesaw up and down. Slip a very thin rubber band over the end of the armature and tack in the end of the block; this will hold it down firmly on the nail. The end of the armature should be one-sixteenth of an inch above screw. Move screw up or down until it is in this position.
The key is made of a piece of sheet brass or copper two and a half inches long, three-eighths of an inch wide. In on end punch a hole to take screw to hold it on base. Three-eighths of an inch from this end give it a slight upward end and three-eighths of an inch from this point give it a downward bend. When screwed on base the longest end should be a quarter of an inch from base.
The base is wood, three inches long, one and a half inches wide and half an inch thick. Half an inch from end bore a hole and drive a screw in, first winding an end of wire underneath it (W). Now screw the lever (L) on the base half an inch from the other end, connecting a wire under it same as before. Glue a common button on end of lever as in sketch. The lever should be one-eighth of an inch above the screw (S) and both the lever and screw should be scraped bright.
To connect up attach a wire from end of magnet wire to the piece under the screw on the key. From the other end of magnet wire, take a piece to the battery; from the other screw on the key take a piece of the battery.
Set the battery close to the sounder. To solder the needle (N) on the armature (A) scrape armature bright and lay on hot stove lid, put a pinch of powdered resin on it and a drop of solder; lay the needle in position; when melted move the armature away from heat until cool. Bore holes with hot nail. Scrape all connections bright. Use round head brass screws.