490 



MAGNETISM AND ELECTRICITY 



been printed from electrotype plates. First a page was 

 set up in type, and then a careful impression of it was taken 

 in wax. Wax is not a good conductor of electricity and so 

 the face of the wax mold was evenly and thinly coated 

 with graphite in order to make it conduct electricity. The 

 graphite-covered mold was then attached to the nega- 

 tive electric pole, as was the lead in Experiment 158, and 

 immersed in the copper sulphate solution. To the positive 

 pole was attached a copper strip. As soon as a layer of 

 copper of the thickness of a calling-card had been deposited 

 on the mold, taking its shape, the newly formed copper 

 plate was separated from the wax impression and was 

 " backed up " with type metal to make it strong enough 

 to be used in the printing press. 



Electromagnet. Experiment 159. Wind several feet of No. 

 20 insulated copper wire around the nail used in Experiment 148 as 

 you would wind thread on a spool. Attach the ends of this wire 



to the poles of a dry cell. 

 Bring the nail thus arranged 

 toward a suspended magnet. 

 Reverse the ends of the nail. 

 Does the nail act as it did 

 before it was placed within 

 the coil of wire connected to 

 the battery ? Bring another 

 nail in contact with its ends. 

 What happens? What has 

 the nail as arranged be- 

 come ? Disconnect one of the wires from the battery and try 

 the test again. Does the nail act as it did when the battery 

 was connected? 



We found that if a nail is placed in a coil of wire connected 

 with an electric battery (Figure 160) it becomes magnetic, 

 but only as long as the connection is maintained. Magnets 



FIGURE 160 



