1059 Main Avenue, Clifton, NJ 07011

The most valuable resources for teachers and students

(973) 777 - 3113


1059 Main Avenue

Clifton, NJ 07011

07:30 - 19:00

Monday to Friday

123 456 789


Goldsmith Hall

New York, NY 90210

07:30 - 19:00

Monday to Friday

A Bell System

A Bell System

Introduction: (Initial Observation)

Electric bell is among the most common electrical equipment found in every house or office. It is used as a door bell, phone ring, fire alarm and can be found in many different shapes and sizes. An electric bell is a simple example of how we can convert electrical energy to mechanical energy and sound waves. The same mechanism used to vibrate a hammer in an electric bell, is used in many other industrial and household equipment. Two well known examples are Electric Shavers, Shearing machines. These two work both work like a buzzer (Another name for Electric Bell), but instead of hammer, a blade will be vibrated.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Adult supervision and help is required in this project.

Information Gathering:

Electric bell is a type of bell operated by an electric current. There are two types of electric bells. One type rings continuously when the switch that controls it is on. This type of bell is commonly used in schools and factories. The other kind, the door chime, rings only once or twice when the switch is turned on–that is, when the doorbell button is pushed.

The parts of a continuously ringing bell include the switch; an electromagnet, a device that acts as a magnet when a current runs through it; and an armature, a movable metal part. A clapper is attached to the end of the armature. Also attached to the armature is a spring that rests against a screw. Wiring runs from the source of electric current to the switch and from the switch to the electromagnet. Another wire runs from the screw back to the source of the electric current. Together, the parts of the bell form an electric circuit.

When the switch is turned on, the current flows through the electromagnet, and the electromagnet attracts the armature. The movement of the armature causes the clapper to strike the bell and the spring to move off the screw. When the spring moves off the screw, the circuit is broken and the current stops flowing. Then, the armature falls away from the electromagnet. When the armature returns to its original position, the spring comes in contact with the screw again and reestablishes the flow of electric current. The process repeats and the electric bell keeps ringing as long as the operating switch is on.

A door chime does not have the spring and screw. Thus, the armature and clapper move only once each time the control switch is operated, and the bell sounds only once. If a second bell is set up for the clapper to strike when it falls away from the first bell, two sounds can be produced for each switch operation.

This drawing shows how an electric bell works:











1st step     The switch is closed
2nd step     A current flows
3rd step     The iron bar is magnetized
4th step    The iron armature is attracted
5th step     The armature moves to the left
6th step     The hammer hits the gong
7th step     The circuit is now broken
8th step     The armature moves back
9th step     The circuit is again complete

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to construct a model of an electric bell and show how it works. Since an electromagnet is the main component of an electric bell, I will also study the relation between the number of loops of wire and the strength of an electromagnet. So my specific question is:

How does the number of turns of wire on the coil affects the strength of an electromagnet?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Since electric bell design is based on electromagnet, voltage, number of loops in the electromagnet’s coil, thickness of the coil and many other factors in this nature may affect the strength of an electromagnet. We will test the effect of the number of loops of wire on the coil, so the variables are defines as follows:

  • Independent variable (manipulated variable) is the number of turns of wire.
  • Dependent variable (Responding variable) is the strength of electromagnet. You may measure the strength of an electromagnet by the weight of small iron nails that an electromagnet can lift.
  • Constants are electrical source (battery and battery voltage); the diameter of wire; type and size of core material; type and size of small nails.
  • Controlled variable is the condition or life of the battery used in this experiment.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. This is a sample hypothesis:

Based on my previous experiments with simple electromagnets, I think that making an electric bell with locally available material will be possible as long as we build a strong electromagnet for that.

I think that as the number of loops of wire increase, a stronger electromagnet will form.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.

Experiment 1: Construct a model electric bell.

Procedure: Basic design of an electric bell consists of an iron bar (armature) that will be attracted by an electromagnet. So I start my project by building an electromagnet and mounting it on a piece of wood. Then I will mount an armature about 3 millimeters above the electromagnet, secured from one side by a hinge or spring tempered sheet of iron. or if my armature is very flexible, I can just mount it using a screw or nail.

Use a piece of wood about 5 inches wide and about 7 inches long. It should be tick enough so you can make a hole on that and secure other components using screws and nails.

To build an electromagnet you can use any screw or nail. It can be about 2 inches tall and and quarter inch diameter. In this experiment we will mount the electromagnet vertical to the base board.
Make a hole and secure the nail or screw in the hole. At least one inch of that should be out to form the electromagnet.
Wrap some paper or tape around the coil as an insulator.
Get some magnet wire and turn it around the nail or screw to form the coil of your electromagnet. Magnet wire is regular copper wire coated with some resins as an insulator. They call it magnet wire because it is used to build electromagnets.

The thickness of magnet wire must be less than 1/32 inches or 0.03 inches. Otherwise the coil gets hot and lots of energy will be wasted as heat. Also the coil must have more than 100 loops. More loops creates stronger electromagnet.

Use a sand paper to remove the insulation of magnet wire at both ends.

You can also use small screws to secure the ends of magnet wire.

Connect the electromagnet to a 6 volt battery. Use a screw driver or nail to see how strong it is.

I found a Popsicle stick and decided to use it as an armature. Since electromagnet can not attract wood, I warped a piece of thin iron sheet around that. I got may metal sheet from a chocolate box and I could cut it using scissors.

After securing Popsicle stick with two screws, I connected one end of the coil wire to the negative pole of battery and the other end to the metal sheet on the Popsicle stick. Then I used another wire and connected one end of that to the positive pole of the battery and used the free end to touch the metal piece on the armature. As soon as this last wire touched the metal sheet buzzer started to work.

The wire on my hand and the metal on the armature together act like a switch that connects and disconnects. I call it contact switch. On this picture I used another piece of metal sheet to hold the wire above the armature.

Note: First I tried using the last metal sheet to be the contact switch, but magnet attracted that and switch never opened. That’s why I finally used another copper wire to be the contact switch.

You will need to repeat such experiments with different number of loops on the coil and different batteries (1.5 volts, 3 volts, 6 volts) and compare and report the results

Another Design


In this design blue dots are nails.
A is an electromagnet made from a bolt and copper coil of more than 100 loops.
B is the armature. It is galvanized or black iron. One end is rolled so we can use a nail. It will be mounted in a way that it stays about ¼ inch away from the electromagnet.
C is contact metal made from copper. It should be touching the armature in a way that it gets disconnected when armature is about half way down. C will be secured on the board using a small screw.
D is a spring to keep armature up. We wouldn’t need it if our armature was spring tempered metal and we could secure it from one end.
E is the gong. Anything that can make sound.

As you will see in the following pictures I have installed the contact metal C almost at the hammer part of the armature. I did that because my board was small and did not have enough room above the armature to mount the contact metal.

In this new design we use a bolts and nuts to build the electromagnet.
Wrap the coil with some tape and mount it on the board using a copper strap.
The armature is a flat sheet of galvanized iron, less than one inch wide and about 8 inches long. I rolled one end so I can use a nail to mount it on the board. I also folded or bended the other end to act like a hammer. Contact switch is a small piece from a copper sheet.
Since this armature also was not a spring, I added a spring to pull it away from the electromagnet. To make sure that the armature will stay close to the electromagnet I also inserted another nail to make sure the armature does not get pulled away and get very far from electromagnet.
An empty can worked as a gong.

Experiment 2: Effect of coil loops on the strength of electromagnets


Electromagnet is the main element in constructing electric bell and many other electrical devices including electric valves, electric switches and electric locks. One important step in designing devices that work with electromagnet is to find out how you can increase the strength of an electromagnet. For example you may suggest any of the following questions:

  • How does the size of the core affect the strength of an electromagnet?
  • How does the material of the core affect the strength of an electromagnet?
  • How does the number of turns of wire in the coil affect the strength of an electromagnet?

An electric bell is no exception. While constructing your electric bell, you may be wondering how many turns of magnet wire on the coil is needed?
If you use only a few turns of wire, the bell will not work. If you use too many turns of wire, other design problems may develop. So you want to know how many turns of wire is sufficient. To make it more general, you need to know how does the number of turns of wire affect the strength of an electromagnet.


  1. Get 10 identical 3-inch bolts and wrap a layer of masking tape or paper over them and number them from 1 to 10.
  2. Wrap wires over the bolts to make electromagnets. Make sure you leave about 1 foot of wire before you start wrapping and leave another foot at the end before you cut the wire. These wire leads are needed in order to connect electromagnets to the battery.
  3. Use the following schedule for the number of turns of wire on each bolt. Wrap 20 turns of insulated wire on the bolt number 1
    Wrap 40 turns of insulated wire on the bolt number 2
    Wrap 60 turns of insulated wire on the bolt number 3
    Wrap 80 turns of insulated wire on the bolt number 4
    Wrap 100 turns of insulated wire on the bolt number 5
    Wrap 150 turns of insulated wire on the bolt number 6
    Wrap 200 turns of insulated wire on the bolt number 7
    Wrap 250 turns of insulated wire on the bolt number 8
    Wrap 300 turns of insulated wire on the bolt number 9
    Do not wrap any wire around the bolt number 10 and keep it as control.
  4. Remove the insulation from the ends of wire on all the coils so they can be connected to the battery.
  5. Connect the electromagnet number 1 to a 6-volt battery. To do this one of the wire leads must touch the positive pole of the battery and the other must touch the negative pole of the battery. Bring the electromagnet over a plate filled with small nails. See how many nails (or how many grams of nails) will be attracted to and will be lifted by your electromagnet.
  6. Repeat the step 1 with the rest of electromagnets from 2 to 9 and record your results in your results table. These will be your first try.
    (By this time your battery may have weakened a little)
  7. Repeat the same in reverse order with each of the 9 electromagnets from 9 to 1 and record your results in your results table. These will be your second try.
  8. Take the average of the number of nails (or weight of nails) lifted in the first and the second try. Write the averages in your results table. Your results table may look like this:
Electromagnet Coil-loops first-try-strength 2nd-try-strength Average Strength
1 20
2 40
3 60
4 80
5 100
6 150
7 200
8 250
9 300
10 (control) 0 0 0 0

If you know the mass or weight of each nail, or you have access to a scale, write the total weight of nails lifted by each electromagnet as its strength. Otherwise, just write the number of nails.

Make a Graph:

When the above results data table is completed, you may use the coil loops column and average strength column to draw a bar graph or a line graph.

Materials and Equipment:

A complete list of material can be extracted from the procedures.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


To calculate the average strength, add the strengths in first try and second try. Then divide the results by two.

If you don’t know the weight of each nail, weight the pack of nails and divide that by the number of nails in the pack.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find additional books about electromagnets.

Following are some web resources: