[Illustration:  Motor-Driven Make-and-Break]

Put the connecting brass bar on the motor shaft with washers fitted tight on each side and slip the other end over the bent end of the wire.  Have the wire plenty long so it can be cut to the proper length when the parts are all in place.  A small round bottle about 1/2 in. in diameter is now fitted in a hole that has been previously bored into the middle of the bottom block and close up to the vertical piece.  This should only be bored about half way through the block.  The wire is now cut so at the length of the stroke the end will come to about one-half the depth, or the middle of the bottle.

Fill the bottle with mercury to a point so that when the motor is running, the end of the wire will be in the mercury for about one-half of the stroke.  Cover the mercury over with a little alcohol.  A No. 14 gauge iron wire is bent and put into the side of the bottle with the end extending to the bottom.  The other end of this wire is attached to one binding-post placed at the end of the bottom block.  The other binding-post is connected to a small brass brush attached to the side of the vertical piece, which is placed with some pressure on the moving wire.  The motor can be run with a current from a separate course or connected as shown on the same batteries with the coil.  The proper height of the mercury can be regulated for best results.  The motor must run continuous if the coil is used for writing code signals, wireless, etc.  —­Contributed by Haraden Pratt, San Francisco, Cal.

** How to Make a Barometer [188]

Atmospheric pressure is measured by the barometer.  The weight of the air in round numbers is 15 lb. to the square inch and will support a column of water 1 in. square, 34 ft. high, or a column of mercury (density 13.6) 1 in. square, 30 in. high.  The parts necessary to make a simple barometer are, a glass tube 1/8 in. internal diameter and about 34 in. long, a bottle 1 in. inside diameter and 2 in. high.  Seal one end of the tube by holding it in the flame of a gas burner, which will soon soften the glass so it can be pinched together with pliers.  Put a little paraffin in the bottle and melt it by holding the bottle over a small flame.  When cool the paraffin should cover the bottom about 1/16 in. thick.  The tube is now to be filled with mercury.  This may be accomplished with a paper funnel, but before attempting to put in the mercury, place a large dish or tray beneath the tube to catch any mercury that may accidentally be spilled.  Only redistilled mercury should be used, and the tube should be perfectly clean before filling.  When the tube is filled to

[Illustration:  Barometer]

within 1 in. of the open end place the forefinger over the hole and tilt the tube up and down so all the air will gather at the finger end.  The filling is continued until the tube is full of mercury.  The glass bottle containing the wax covered bottom is now placed over the end of the tube and pressed firmly to insure an airtight fit with the tube.  The bottle and tube are inverted and after a few ounces of mercury are put in the bottle the tube may be raised out of the wax, but be careful not to bring its edge above the surface of the mercury.