** Door-Opener for Furnace [83]

The accompanying diagram shows an arrangement to open the coal door of a furnace.  When approaching the furnace with a shovelful of coal it is usually necessary to rest the shovel on the top of the ash door, while the coal door is being opened.  With my device it is only necessary to press the foot pedal, which opens the door.  After putting in the coal, pressing the pedal closes the door.  The pulley in the ceiling must be placed a little in front of the door, in order to throw the door open after lifting it from the catch.  A large gate hinge is used to hold the pedal to the floor.  —­Contributed by Edward Whitney, Madison, Wis.

[Illustration:  Furnace Door Opener]

** How to Make an Efficient Wireless Telegraph [84] By George W. Richardson

A simple but very efficient wireless telegraph may be constructed at slight cost from the following description: 

The sending apparatus consists of nothing but an induction coil with a telegraph key inserted in the primary circuit, i. e., the battery circuit.  This apparatus may be purchased from any electrical-supply house.  The price of the coil depends upon its size, and upon the size depends the distance signals can be transmitted.  If, however, one wishes to construct his own coil he can make and use, with slight changes, the jump-spark coil described elsewhere in this book.  This coil, being a 1-in. coil, will transmit nicely up to a distance of one mile; while a 12-in. coil made on the same plan will transmit 20 miles or even more under favorable conditions.

Change the coil described, as follows:  Insert an ordinary telegraph key in the battery circuit, and attach two small pieces of wire with a brass ball on each, by inserting them in the binding-posts of the coil as shown at B B”.  Of these two terminal wires one is grounded to earth, while the other wire is sent aloft and is called the aerial line.  This constitutes all there is to the sending apparatus.

Now for the receiving apparatus.  In the earlier receiving instruments a coherer was used, consisting of a glass tube about 1/8-in. diameter, in which were two silver pistons separated by nickel and silver filings, in a partial vacuum.  This receiver was difficult of adjustment and slow in transmission.  An instrument much less complicated and inexpensive and which will work well can be made thus: 

Take a 5-cp. incandescent lamp and break off the tip at the dotted line, as shown in Fig. 5.  This can be done by giving the glass tip or point a quick blow with a file or other thin edged piece of metal.  Then with a blow-torch heat the broken edges until red hot and turn the edges in as seen in Fig. 6.  Remove the carbon filament in the lamp and bend the two small platinum wires so they will point at each other as in Fig. 6, W W. Screw the lamp into an ordinary wall socket which will serve as a base as in Fig. 7.  Make a solution of 1 part sulphuric acid to 4 parts of water, and fill the lamp about two-thirds full (Fig. 7).  This will make an excellent receiver.  It will be necessary to adjust the platinum points, W W, to suit the distance the message is to be worked.  For a mile or less the points should be about 1/16 in. apart, and closer for longer distances.