SCIENCE AND PRACTICE. 417 



circuits, therefore, the quantity flowing into the cable or out 

 of it may be measured, if the contacts follow each other with 

 sufficient rapidity, by a constant deflection. 



The "Wippe" consists of two permanent steel bar-magnets 

 supported at right angles to each other upon a vertical shaft, 

 turning on points in stone bearings. Around these magnets 

 are two coils of wire at right angles to each other, and whose 

 ends are connected with a commutator, turned by the shaft in 

 such a way that when the magnet system is deflected a 

 certain distance by the current, the latter is then reversed and 

 the deflection continued in the same direction till the magnets 

 have made half a revolution, when the first current direction 

 obtains again, keeping the magnets rotating with an immense 

 velocity. For this a driving battery, separate from the 

 experimental battery, is required. 



The shaft carries a small metallic eccentric, e (Fig. 187), 

 which, turning between the prongs of a fork, //, rocks the 

 beam b b, lifting alternately the springs s and s / from the 

 contact anvils c and c r . The springs, anvils, and beam are 

 insulated from each other and connected by leading wires 

 with convenient terminal screws. 



The terminals of the Wippe may be connected in various 

 ways to fulfil the same duties. If 



both the ends of the cable are at /^~7OOV7wVX?j^\G 

 hand, they may be connected between [ CD 



the springs s and s f , as in Fig. 188, 

 the galvanometer G being inserted 

 in the same circuit, while a battery, 

 }, is connected between earth and c' ; 

 und c is put to earth. 



When the beam b is in the posi- 

 Mon shown in the figure, whatever 

 blectricity is in the cable will be 

 discharged, by way of s, c, and earth. 

 When the beam places itself in the Fl - 188 - 



opposite way, as shown by the dotted lines, the end of the 

 cable which was before on earth will now, in turn, be insu- 

 lated, and the galvanometer end put, through / and </, in 



E E 



