Assembling the bell, the relay and the electric motor

An electromagnetic relay is a device that uses a weak current to switch a high-power circuit on and off from a distance. Its main part is an electromagnet whose winding is in the control circuit. When the control circuit is closed, current flows through the winding, the armature is drawn to the core and the contacts of the working circuit close; when the circuit opens, a spring returns the armature and breaks the working circuit. A relay can be controlled by changes in temperature, pressure or illumination: in a fire alarm, for example, thermistors lose resistance as they heat, the current grows, the relay operates and a bell rings. The commutator electric motor changes electrical energy into rotation. Its fixed part (stator) makes the magnetic field and the rotating rotor (armature) carries windings. Current reaches the rotor through brushes and the commutator; the commutator reverses the current in the rotor every half turn to keep it turning. In a small DC motor, swapping the battery poles reverses the direction of rotation. An electric-car model consists of a motor, battery, switch, gear drive and wheels. The model runs from a 1.5–3 V battery; if the motor stalls it heats up, so disconnect immediately on a short circuit or jam. Small parts (screws, wheels) are not put in the mouth.