Connect your homemade electric meter, in series, with a small coil of bare wire and a dry cell. Heat the bare-wire coil, and the current will decrease, as shown by the lessening deflection of the compass needle. The resistance of the wire has been increased by heating, so that less current is allowed through. Now substitute a beaker of cold, salt water for the coil, and connect the wires to metal plates dipping in the liquid. In this case, heating the salt water will indicate a greater flow of current on your meter, owing to lessened resistance of the conducting solution. This illustrates a general rule of electricity- that heating a solid conductor usually increases its resistance, while heating a liquid conductor, or electrolyte, usually diminishes its resistance. No pencil resistance is needed, as this is an amperage test.
Heating the coil of wire increases its electrical resistance. At the left, a pencil "lead" included in the circuit of the homemade meter gives it wider range when used for testing voltages.
THIS NOVELTY MOTOR is made with a oar magnet and a box top partly filled with mercury and supported on a tumbler. Suspend copper wire from the hooked end of a stiff wire bent as shown and attached to one pole of a dry cell. Dip a wire from the other pole into the mercury. Repelled, the suspended wire spins around the magnet; reverse connections, and it spins the other way.
Assemble bolt wound with bell wire, strip cut from a tin can, and a wood screw. The adjustable screw should just touch the strip. Current from dry cells makes bolt a magnet, and draws strip toward it. This breaks the circuit, and flexible strip springs back.
Lay a pencil lead across the edges of two razor blades connected to dry cells and earphones as shown. Any slight vibration-the ticking of a watch, or passage of breath across the lead- gives a thunderous response in the headset, by changing the electrical resistance of the lead (really carbon) at its contacts with the blades. Similarly, in telephone transmitters and microphones, the voice compresses and loosens carbon granules.
Taking a 4' hurdle, one obstacle on a course scientifically designed to help make boys physically fit.
Junior commando course