Activity 4
Making the Connection
Background Information
A telegraph has a key for the sender to push to complete a circuit. Holding the key down for just an instant specifies a “dot.” Holding the key down for some longer time specifies a “dash.” Usually the key is spring-loaded so that when it is not pushed, it pops open and the circuit is broken. Although the sending part of the circuit is fairly standard, at the receiving end there are many possible designs. The first telegraph receivers were made by winding a coil around a compass. When the key was held down, current ran through the coil and deflected the compass. The distance of the deflection identified what was sent as either a “dot” or a “dash.” The main disadvantage of this method is that the compass swings back and forth for a few seconds when the circuit is opened again, and during these seconds no further signal can be transmitted. Another possible receiver is an electromagnet. The electromagnet can pick up paper clips, as in Activity 3, each
time the circuit is closed. When the circuit is opened, the electromagnet drops the clips. There are some challenges in this kind of receiver. First, both the paper clips and the nail are magnets even after the current is turned off. In fact, a paper clip may stick to the nail after the current stops flowing. To prevent this, students might use large paper clips or, more elegantly, tie something to the paper clip to gently pull it away from the electromagnetic when the current stops. In a different design, shown in the drawing, holding down the key produces a buzzing sound at the receiver. The curved wire at the top touches the elastic metal strip but is not fastened to it.
Current turns on the electromagnet and attracts the elastic piece of metal. But when the metal moves toward the electromagnet, it breaks the circuit, so the electromagnet shuts off and the metal moves back where it was before. Again the circuit turns on, the metal moves toward the electromagnet, and so on. The constant back-and-forth movement of the flexible piece creates an distinctive buzzing sound. A long buzz is a “dash,” and a short buzz is a “dot.”
Making the Connection
Background Information
A telegraph has a key for the sender to push to complete a circuit. Holding the key down for just an instant specifies a “dot.” Holding the key down for some longer time specifies a “dash.” Usually the key is spring-loaded so that when it is not pushed, it pops open and the circuit is broken. Although the sending part of the circuit is fairly standard, at the receiving end there are many possible designs. The first telegraph receivers were made by winding a coil around a compass. When the key was held down, current ran through the coil and deflected the compass. The distance of the deflection identified what was sent as either a “dot” or a “dash.” The main disadvantage of this method is that the compass swings back and forth for a few seconds when the circuit is opened again, and during these seconds no further signal can be transmitted. Another possible receiver is an electromagnet. The electromagnet can pick up paper clips, as in Activity 3, each
time the circuit is closed. When the circuit is opened, the electromagnet drops the clips. There are some challenges in this kind of receiver. First, both the paper clips and the nail are magnets even after the current is turned off. In fact, a paper clip may stick to the nail after the current stops flowing. To prevent this, students might use large paper clips or, more elegantly, tie something to the paper clip to gently pull it away from the electromagnetic when the current stops. In a different design, shown in the drawing, holding down the key produces a buzzing sound at the receiver. The curved wire at the top touches the elastic metal strip but is not fastened to it.Current turns on the electromagnet and attracts the elastic piece of metal. But when the metal moves toward the electromagnet, it breaks the circuit, so the electromagnet shuts off and the metal moves back where it was before. Again the circuit turns on, the metal moves toward the electromagnet, and so on. The constant back-and-forth movement of the flexible piece creates an distinctive buzzing sound. A long buzz is a “dash,” and a short buzz is a “dot.”