ELECTRO-MAGNETIC INSTRUMENTS. 



21 



from a loop, and capable of free motion ; its lower end is 

 amalgamated, and dips into a small cistern of mercury ; the 

 cups, a and b, are filled also with mercury, and through 

 them the electrical current is passed down the loose wire ; 

 no motion of this wire is perceptible, until a horse-shoe 

 magnet is placed in a horizontal position, on the basis, with 

 its poles enclosing the wire, when il is instantly urged 

 either forwards, towards c, or backwards, towards d, accord- 

 ing to the position of the poles, and the direction of the 

 current. In either case it is thrown out of the mercury, 

 and the circuit being thus broken, the effect ceases, until 

 the wire falls back again by its own weight, into the mer- 

 cury ; when the current being re-established, the same 

 influence is again exerted, the phenomenon is repeated, and 

 the wire exhibits a quick succession of vibratory motions. 



Price, $2.00. 



Fig. 439. 



Barlow's Spur Wheel. (Fig. 439.) This instrument is 

 mounted on a mahogany base, having a small trough for mer- 

 cury, and supporting a small brass figure, to which an arm 

 is attached, branching off and bending downwards, being 

 divided at its extremities into two branches, between which 

 a spur-wheel, having a number of rays, is made to revolve 

 in two centres. A horse-shoe magnet is laid on the base, 

 with its poles inclosing the spur-wheel. One of the poles 

 of the battery is to be connected with the mercury in the 

 trough, and the other to the arm supporting the wheel, or 

 with mercury cups attached to these parts for the purpose. 

 The wheel, being constructed so as to turn round freely, will 



