488 



MAGNETISM AND ELECTRICITY 



lamps are also made with metal filaments. Only two 

 metals, tantalum and tungsten, have been found that will 

 withstand the intense heat. Incandescent lamp filaments 

 made from these metals are necessarily much longer and 

 thinner than the carbon filaments, and are therefore more 

 easily broken. But their great advantage lies in the fact 

 that they use only about one third the amount of current 

 in giving the same light. A tungsten filament will with- 

 stand much heavier jarring when it is hot than when cold. 

 It sometimes happens that a lamp has imperfections that 

 render it dangerous to handle carelessly. If one touches 

 the metal part of such a lamp when it is in use, especially 

 with wet hands, one is likely to receive a severe shock. These 

 shocks have sometimes proved fatal. To avoid such possible 

 danger one should touch only the hard-rubber switch in 

 turning a light on or off. Especial care should be taken 

 when the hands are wet, because moisture is an excellent con- 

 ductor of an electrical current. 



Electroplating. Experiment 168. Almost fill a dish with a 

 strong solution of copper sulphate (blue vitriol). Across the dish 



^_ and a little distance apart, 



place two parallel wooden 

 rods. Carefully clean with 

 fine sandpaper a strip of lead 

 and a strip of copper. Punch 

 a hole in an end of each strip 

 and attach to each strip two 

 or three feet of fairly heavy 

 copper wire . Pinch the wires 

 firmly on to the copper and lead at the points of connection. Sus- 

 pend a strip from each of the rods by winding the wire once around 

 the rod. Attach the wire from the copper to the positive pole of a 

 battery and the wire from the lead to the negative pole. A copper 

 plate will be deposited on the lead. 



SIMPLE APPARATUS FOR ELECTROPLATING 



