318 



LABORATORY GUIDE IN PHYSIOLOGY. 



Materials and Construction. (1) A soft iron electro- 

 magnet (m) with soft iron armature (a), as shown in 

 A. A machinist or electrician can construct these from 

 strictly pure, soft Swedish iron. 



(2) No. 24 double silk covered copper wire, to be wound 

 as indicated in A (x to y). The wire should be wound 

 in three layers and when the winding is complete it 

 should present the appearance shown in Fig. 62 B, m'. 



(3) From fiber board or from wood one may construct 

 such a lever and magnet support, as shown in Fig. 00 

 B. The lever (1) is pivoted at f; the block a' bears 

 the armature; the counterpoise (w) may be adjusted 



FIG. 62. 

 FIG. 62. The Chronograph. 



so as to make the part of the lever at the right of f 

 slightly heavier than that at the left, so that when no 

 current is flowing through the electro-magnet the 

 armature is lifted from the magnet. 



(4) A check (c) rests upon an adjustable screw (s) and 

 limits the excursion of the lever. 



(5) A straw may be fixed with wax to the end of the 

 lever and a tracing point (p) of parchment paper 

 slipped into the straw. 



(6) The wires from the clock or the chronograph sys- 

 tem are connected at x' and y'. 



