THE ELECTRIC BELL. 735 



these cases to replace the two outer screws in fig. 370 by binding 

 screws, which admit of more readily establishing connection with 

 any other apparatus. 



The small sparks produced whenever contact is broken cause 

 the metal to become gradually oxydised. at the parts which touch 

 each other, unless they are made of some metal which does not 

 become oxydised when heated in air. Such a metal is platinum, 

 and if possible those parts which come into frequent temporary 

 contact during the working of the apparatus, viz., the end of the 

 spring / 2 and of the screw s in fig. 369, and also the ends of the two 

 strips in fig. 370, should be made of platinum. If this cannot be had 

 the parts mentioned must be frequently looked after, and kept as 

 bright as possible by the use of emery paper. For a mere demon- 

 stration of the action of the apparatus the use of platinum would be 

 unnecessary ; but if the bell is to be permanently used, it is best to 

 solder small strips of platinum foil upon that portion of the end of 

 / 2 which is opposite to the screw s, and similar strips must be 

 soldered upon those portions of the two strips in fig. 370 which 

 come into contact when the spring is pressed down. The screw s 

 is also easily provided with a platinum point by drilling a fine hole 

 into the end of it, and soldering a short pointed piece of thick 

 platinum wire into the hole. 



Platinum itse^ fuses with great difficulty, but it has the property 

 of forming with other metals for example, with soft solder alloys 

 which are comparatively very fusible. In soldering platinum care 

 must therefore be taken not to apply too great a heat. 



52. Magnetism It has been shown previously (see 



page 643) that one of the effects of the electrical dis- 

 charge through a spiral which is coiled round a small 

 bar or needle of steel is to render the needle magnetic ; 

 it acquires the property of attracting particles of iron. 

 This property is not acquired temporarily, as in the case 

 of the electro-magnets described in the preceding 

 article, it does not cease with the current which pro- 

 duced it, but the steel bar, after the experiment, has 

 permanently acquired the property of attracting iron. 

 If the bar of steel is placed for a short time inside the 

 spiral of fig. 35-3, and submitted to the action of the 



