Let's find out how bells and buzzers work, what different kinds there are, the different ways you can control them, and how you can put them to work for you. You'll find that buzzers and bells can help you with your 4-H projects, and with the proper controls, can be your eyes and voice in a dozen places at once. Why They Buzz or Ring--Electromagnetism If we were to look at an electric bell with the cover off, we'd find that it would be very much like Figure 1. A push on the button, which is just a switch that is normally held "open" or off by means of a spring, sends the current from the battery or transformer through the circuit. [Illustration: Figure 1] You will see that the current passes first through two small coils of wire, and each coil has at its center a piece of soft iron called the core. When the current is on, the core becomes magnetized and attracts another piece of iron called the armature with its clapper attached. This action rings the bell, but it also breaks the current by pulling the spring away from the screw on its return to the power supply. With the power off, the electromagnet lets the spring return the armature to its normal position, contact is made again, and the cycle starts all over again--just as long as you continue to push on the button. Buzzers work exactly the same way, except that they do not have a bell and depend instead on the vibration of the armature for a noise that's not as loud or as musical. Gongs or chimes, that strike only once when the button is pushed, are made by connecting the armature with the screw by means of a flexible wire. A Special Kind of Electricity Most buzzers and bells work on a much lower voltage than you normally find in the wires in your house. Some are made to work at 6 volts, others at 10 volts, and still others at slightly higher voltages. You can get these low voltages by using one or more batteries, or by using a transformer connected to your house current. Most bells and buzzers are now powered through transformers. How to Control Them The push button is the most common means of control. You can use one button to control several bells, or several buttons to control one bell, or have several buttons control several bells. Because low voltage is used, adding extra buttons is simple, inexpensive, and safe. Buzzers and bells can also be controlled by: _clocks_, as in the interval timer on an electric range or in a school class bell system; _temperature detectors_, as in a fire alarm or freezer alarm; _door and window trips_, as in a one-man repair shop or in a burglar alarm; and _treadles_, as in the driveway of a service station. [Illustration: Figure 2] Pick the Right Bell or Buzzer Some of the many different types of bells, and various ways of controlling them are suggested in the table below. Just remember that no matter what the job or conditions, you can probably find a bell or buzzer and controls that suit your need.