48 LABORATORY EXERCISES 



a. We have already tested the effect of a wire carrying a 

 current upon the magnetized needle or compass. Try it again 

 with the sal ammoniac cell, but use covered ("insulated") 

 wire. 



6. Coil the insulated wire around a thick pencil, giving it 

 about 10 turns. Hold the coil of wire, with a current flowing 

 through it, in some iron filings or small tacks. Do any cling 

 to the coil? 



c. Into the coil of wire put a bar of soft iron. A large wire 

 nail will do, if you first heat it red hot, and then let it cool 

 slowly. Bring the soft iron "core" of this "electromagnet" 

 near some iron filings or tacks. Are more, or less, picked up than 

 when the coil alone was used? Now "break the circuit" by 

 disconnecting one of the wires from the cell, or by separating 

 the wires that connect the zinc and the carbon. What takes 

 place? 



d. Again coil the insulated connecting wire around a pencil, 

 winding it in about 50 turns, or as many as the length of the 

 wire permits. Instead of doing this you can use a coil, already 

 prepared and having a large number of turns. Into the coil 

 put the soft iron core, and let the current flow. How does the 

 ability of the electromagnet to pick up iron nails, etc., compare 

 with its strength when the coil is small? 



What, then, is the advantage gained by the use of a coil of 

 many turns? 



EXERCISE 47 

 SHADOWS 



Apparatus and Materials. Ordinary candle and "birthday" candle, 

 each about 8 cm. (3 in.) high, two cardboard squares, upright object, 

 such as two spools; kerosene or electric lamp. 



a. Take an ordinary candle and a small "birthday" candle, 

 each about 8 cm. (3 in.) high, and fasten them to cardboard 



