PERMANENT MAGNETS AND THEIR USES (XIX-1)

The problem. Permanent magnets are now used for compasses, tack hammers, telephone magnets, toy magnets, ammeters, voltmeters, and magnetos. What characteristics make such magnets useful in these devices?

Ring

What to use. stand; burette clamp; two bar magnets; box of scrap iron, tacks, iron filings and other materials; stick of wood; string; plate of glass; wood supports; and an iron pan.

What to do. 1. Put the bar magnet into the box of small objects. Remove the magnet and make a list of the substances that are attracted to it.

Find the parts of the magnet which attract the most filings. 2. Place a small nail on a piece of glass and move the magnet under the glass. Try this with cardboard and iron.

3. Suspend two bar magnets as shown in figure 36. Mark the ends which point to the north. Remove one magnet and bring the north poles near each other. Try the other combinations suggested below. Record your observations.

4. Lay the bar magnet under the glass as shown in figure 36. Place a piece of paper over the glass and sift iron filings over the paper. Tap the paper gently until lines of filings appear. Make a drawing of the magnet and the lines. Mark each pole as north or south, as the case may be.

5. Place two north poles near each other under the glass. Repeat 4.

6. Repeat 5 with a north pole and a south pole near each other.

Questions. Where are the poles of a bar magnet? Where is the attraction strongest? weakest? Will a magnet exert an influence through all substances? What is the rule for magnetic attraction and repulsion? How is a compass made? What is the condition of the region about a magnet as shown by the iron filings?

Suggestions for report. Correct all errors, then file your results and drawings in the notebook.

NORTH TO NORTH SOUTH TO SOUTH NORTH TO SOUTH

SOUTH TO NORTH

Reference work. Read sections 231 to 234.

Optional problems. Can you make a compass out of a needle or a watch spring? Make a map of the magnetic field about any of the magnets suggested above.

HOW DOES AN ELECTROMAGNET OPERATE? (XIX-2)

The problem. The electromagnet, discovered by Joseph Henry, has proved to be a highly useful device. The telephone, telegraph, electric bell, door opener, arc lamp, motor, and dynamo would not be possible without this mechanism. In learning how to make and operate an electromagnet we shall begin by studying the magnetic action of the coils of an electric bell.

What to use. New dry cell, push button or switch, four feet of No. 18 copper magnet wire, iron filings, compass, large nail, and buzzer or electric bell.

What to do. 1. Connect an electric bell or buzzer to the dry cell as shown in figure 116 of the text. Remove the cover and study the action as follows: Note the insulation, set screw, spring, and magnet coils. Send a current through the circuit and watch the action. Write a brief explanation telling how and why the bell works.

2. Arrange the apparatus as shown in figure 37 and perform the experiment given in section 234 of the text.

3. Test a large nail for magnetism by the use of iron filings and a compass. If the nail is magnetized, reject it and try another. Wind 20 or 30 turns of the wire on the nail. Send a current through the coil and test the nail for magnetism with iron filings

and a compass. Mark the north pole and the direction of the current through the coil. Reverse the dry-cell connections and again test the poles of the nail.

4. Remove the nail from the coil and repeat 3. Write a rule for telling the relation between the direction of the current in the coil and the north pole of the coil.

Questions. Explain why the electric-bell clapper vibrates when the circuit is closed. What is the purpose of the set screw? How are the magnet coils connected? What causes the spark? Will the bell give an electric shock? How would you show that a current in a wire produces a magnetic effect? Can you state a rule from 2? How does a current in a coil affect the iron core? What is the effect of reversing the current? removing the iron core? How can you find the north pole of a coil when the direction of the current is given?

Suggestions for report. After a class discussion of the experiment write correct statements for all explanations and questions given above.

Reference work. Read sections 233 to 237. Make a list of all the devices you have seen in which an electromagnet is used.

Optional problems. Arrange an electric bell to be rung from either of two push buttons. Connect one dry cell, two bells, and two push buttons to form a street-car-bell circuit. Make a diagram to explain the electric circuit of an elevator annunciator.