MAGNETISM AND ELECTRICITY 



403 



Take for illustration Oersted's fundamental experiment of 

 tbe deflection of a magnetic needle by the current ; it is per- 



FIG. 222. Oersted's Experiment 



fectly easy to arrange an entire fixed apparatus, and project 

 it by the vertical method. But the truly scientific demon- 

 strator will rather prefer 

 to place the bare needle 

 first in the focal plane, and 

 bring over his wire inde- 

 pendently, as in fig. 222, 

 simply because his ex- 

 planation will thus be 

 better understood, which is 

 his object. 



Ampere's experiments 

 showing the attraction or 

 repulsion of parallel cur- 

 rents may be shown by 

 any of the usual apparatus 

 made on a small scale ; but 

 here again the same rule 

 applies, and it is better to 

 confine the set apparatus 



FIG. 223 



to the movable portion, 



and bring up the other current independently, as in fig. 223, 

 which is an arrangement by Prof. Forbes. A small arrow 

 may be fixed to the wire which is held; and in this way 



