The telegraph was one of the earliest devices to harness the power of electricity to communicate. It revolutionized long-distance communication. The telegraph key was an essential component of this communication device. Through this activity your students will build a working telegraph key and learn about simple circuits and some basic aspects of conductivity.
Overview and Background
In this activity students will:
i) Learn how to build a simple circuit with a switch;
ii) Learn about the basic aspects of conductivity.
Background and Additional Information
Early forms of long-distance communication were based primarily on sending codes and
symbols using visual indicators such as smoke signals. The discovery of electricity made it possible to transmit messages along electrical lines. This spurred considerable research, culminating in the invention of the electric telegraph. The electric telegraph was more versatile and reliable than visual indicators. It also provided instantaneous contact, revolutionizing long-distance communications.
A telegraph key is used to transmit messages in an electrical telegraph. Telegraph keys come in many different models. The one seen here can be found in Ingenium’s collection of artifacts and is called a Camelback because of the hump-shaped look of the lever. It is made out of brass and was used in a telegraph office in Metcalfe, Ontario, which opened in 1870 and closed in 1912.
Lesson Plan and Activity
Assembling the Telegraph Key
For this activity we will use the letters found in Image 1 of the Main Image viewer (at the top of this page) to indicate the various screws and discs.
Build the Base
1) Fasten the curved handle into the O-shaped base of the telegraph key with Screw A. Make sure that the handle curves toward the base (Image 2 in the Main Image viewer).
2) Twist Discs C and D onto their respective screws. Fasten each of the disc-screw combinations into the side of the base (Image 3a and 3b in the Main Image viewer).
Build the Lever
1) Fasten Disc B onto its screw. Fasten the disc-screw combination onto the lever of the telegraphy key (Image 4 in the Main Image viewer).
Operating the Telegraph Key
Now it’s time to add parts to the telegraph key to make it operational.
1) Take the two brass fasteners and bend the tip of the longer arm to create a small tab on each (Image 5 in the Main Image viewer).
2) Glue one of the brass fasteners to the hammer of the lever (Image 6 in the Main Image viewer). Glue the other brass fastener to the anvil of the base (Image 7 in the Main Image viewer). In each case, separate the arms of the brass fastener so that they lie on either side of the object to which they are glued.
3) Attach the lever to the base by inserting the pointed ends of the lever into Screws C and D (which were fastened to the base earlier).
4) Wrap the rubber band around the lever and the base at the end furthest away from the brass fasteners. This will make the telegraph key bounce back up when used (Image 8 in the Main Image viewer).
5) Attach an alligator clip to the tab on each brass fastener.
6) Attach the other end of one of the alligator clips used in step 5 to positive wire of the 9V battery holder (this is usually the red wire). Attach the other end of the other alligator clip used in step 5 to positive terminal of the light/buzzer (in a LED light, this is the longer lead/wire).
7) Use a third alligator clip to attach the negative terminal of the light/buzzer to the negative wire of the 9V battery holder (this is usually the black wire).
8) Clip the 9V battery into the holder (Image 9 in the Main Image viewer).
9) Optional: If you are using a LED light, the telegraph key can cause it to heat up and burn out if the key is held down for an extended period of time. To prevent this, you can add a resistor to your circuit. A resistor between 200-300 Ω for a 9V battery should be sufficient (see step 10 for an optional calculation). In addition, you will need another insulated wire with alligators clips (we’ll call this “Wire 4”). To add a resistor:
a) Locate the wire with alligator clips that is attached to the positive wire of your battery.
b) Unhook one of the alligator clips of this wire.
c) Attach this alligator clip to one end of the resistor.
d) Attach the other end of the resistor to one of the alligator clips of Wire 4.
e) Attach the other alligator clip of Wire 4 to where you unhooked the wire in step b.
(Image 10 in the Main Image viewer)
10) Optional: If you wish to determine the exact resistor needed for your LED light, use the equations below:
a. Voltage (in V) = Voltage of battery (in V) – Voltage of the LED light (in V)
b. Resistance of resistor needed (in Ω) = Voltage (in V) ÷ Maximum current of LED light (in A)
You can usually determine the LED light’s voltage and maximum current from its packaging.
11) Your telegraph key is now ready to use! You have now built a simple circuit in which the telegraph key functions as a switch. When you press down on the telegraph key, this completes the circuit, which is why the buzzer beeps (or the LED light, lights up).
To further explore the concept of conductivity ask your students what they think would happen if different types of material are introduced into the telegraph circuit. Some things to try: a piece of paper, a piece of string and a coin. Then introduce these new materials into the circuit using an insulated wire with alligator clips.
1 - 9V battery holder^
1 - 9V battery^
1 - Glue gun
1 - Printout of telegraph key parts
4 - Insulated wires with alligator clips
1 - Rubber band
2 - Brass fasteners
1 - Electric buzzer or LED light bulb^^
1 - 200-300 Ω resistor (optional)
^ Note: You can also use a different battery and holder (eg. AA)
^^ Note: You can use anything that indicates the presence of an electrical current. Make sure that whatever you use can handle the voltage of your battery.