92 ELEMENTS OF LABORATORY WORK 



a key, and observe what occurs when the circuit is completed and also 

 when it is broken. 



11. Introduce wires of different thickness and different lengths (e.g. 

 Nos. 16, 20, 32, and 40 B. W. G.) into a given circuit together with a 

 galvanometer, and observe the effects. 



12. Give some demonstrations of Ohm's law viz. that the 'strength 

 of current ' varies directly as the electromotive force ' and inversely as 

 the resistance. 



13. Construct a coil which possesses a resistance of 100 ohms by 

 winding upon a reel some No. 32 silk-covered wire, German-silver pre- 

 ferably. Compare the wire before winding with a standard 100-ohm 

 coil. After adjustment wind doubly from the middle, so that the coil 

 may not possess magnetic properties (in this case there will be an 

 equal number of turns in each direction, and, therefore, magnetic equi- 

 librium). Solder the ends to terminals and coat with paraffin. The 

 coil will probably be slightly inaccurate when carefully tested, but the 

 experience will be valuable. Kemember that the wire twisted round a 

 binding-screw must not be counted as offering resistance. 



14. Observe the effect of joining the terminals of a galvanometer 

 by thick and thin wires while it is connected with the same battery. 

 How would you prove that, when there is a choice of paths for the 

 current, as in this case, the quantity along each path varies inversely as 

 the resistance ? 



