=261. The electric bell= (see Fig. 241), consists of an electromagnet, _M_, a soft iron _armature_, _A_, attached to the _tapper_, _T_, and a post, _R_. When no current is flowing a spring at _S_ holds the armature against the post _R_. When current flows through the helix, its core becomes magnetized and attracts the armature, drawing it away from the post, _R_, and causing the tapper to hit the bell. Drawing _A_ away from the post, however, breaks the circuit at _R_ and the current stops. The magnetism in the core disappears releasing the armature, which is then pulled back by the spring _S_ against the post _R_. This completes the circuit and the process repeats itself several times a second as long as the current flows. [Illustration: FIG. 242.--Magnetizing by the discharge of a Leyden jar.] =262. Static and Current Electricity Compared.=--The likeness between a discharge of static electricity and an electric current may be shown by winding a coil of insulated wire about a glass tube which contains a steel needle. If a Leyden jar (see Fig. 242) is discharged through the coil the steel needle is usually found to be magnetized, showing that the discharge of the static electricity has a magnetic effect similar to that of an electric current. Sometimes a given end of the needle has a north pole and at other times a south pole. This is believed to indicate that the charge of the Leyden jar is _oscillatory_, and that in different discharges sometimes a surge in one direction and at other times a surge in the reverse direction has been most effective in magnetizing the needle. Compare this action with that described in Art. 233. Important Topics 1. Right-hand rules, for conductor, for helix. 2. The electromagnet, two forms, where used? 3. Likeness between static and current electricity. 4. The electric bell, parts, action. 5. The telegraph, key, sounder, relay. Exercises 1. What is the difference between an electric charge and a current? 2. How can a magnetic effect be produced from an electric charge? 3. What is a magnetic field? Give two evidences of a magnetic field about a current in a wire? 4. A current is flowing north in trolley wire, what is the direction of the magnetic field under the wire? Explain. 5. What would be the result if a hard steel core were placed in the electromagnet? Explain. 6. If the north-seeking pole of a helix is facing you, does the current in the coils before you move in a clockwise or in a counter-clockwise direction? Explain. 7. A helix is placed horizontally with its north-seeking pole toward the north. Does the current in the wire at the top of the helix move east or west? Explain. 8. State at least six conditions any one of which will put an electric bell circuit out of commission. 9. If one desires to insert a battery into a telegraph circuit already in operation, how will he determine the direction of the current in the wire?