AN EXPERIMENT. 2OI 



next wound round the strip of zinc and its copper covering a piece of 

 platinum wire, the wire becoming red through the electricity developed in 

 the pile. The dimensions of this platinum wire were extremely small ; it 

 was only TTTTRT of an inch in diameter, and sV of an inch in length. By 

 reason of its small dimensions it could not only be reddened, but fused 

 by the little battery. 



Thus Wollaston's friend, who was a witness of the experiment, was able 

 to light a tinder at the red wire. In this little battery of Wollaston's 

 the copper enveloped the strip of zinc in every part; that is to say, the 

 negative element was on a higher surface than the positive metal. 



After considering Electricity, it is not impossible to approach the study 

 of Magnetism, and even to construct a mariner's compass. .We shall find the 

 method of doing so by borrowing an interesting passage from the " Magasin 

 Pittoresque" Let us take a small cork and pass through it an ordinary 

 knitting-needle (fig. 203), which we have already magnetized by placing it 

 N.S., rubbing it gently, and always in the same direction, with one of those 



Fig. 203. A simple compass. 



little iron magnets with which children amuse themselves. After the needle 

 has been passed through the cork, we also fix into it a sewing-needle, or 

 rather a pin, the point of which rests in one of the little holes in the upper 

 part of the thimble. In order to balance the magnetised needle, we thrust a 

 match into both sides of the cork, as shown in the illustration, and fasten to 

 the ends of each a ball of wax. Thus the needle, the balls, and the pin are 

 all balanced at once, so that the contrivance has the appearance of the illus- 

 tration. 



As it is very important that with such a sensitive instrument any 

 agitation of the air should be avoided, the thimble must be placed at the 

 bottom of a common earthen pan, B D T, which should be covered over with 

 a piece of glass, v. To graduate the compass a circle is described on a piece 

 of paper. On this dial we trace the divisions sufficiently close only at the 

 north extremity of the needle, and the paper is fixed underneath, as in 

 fig. 203. Then we fix a piece of wax at the end of the match pointing N., 

 opposite the northern extremity of the needle inside the basin. In this way 

 we have a very useful and inexpensive compass. 



We may also magnetize a fine sewing-needle, and grease it by rubbing 



