312 Dr Wilson on the Electrical Magnetic Bell. 



fastened two wires, passing through the thickness of the 

 glass, which on the upper side could be connected with the 

 terminations of the electro-magnetic coils, and on the lower 

 with wires from the galvanic battery. This arrangement 

 seemed very satisfactory. The polish and smoothness of the 

 plate-glass secured the bell-jar fitting tightly ; the mode in 

 which the wires entered prevented them concealing the 

 more important parts of the bell ; the transparency of the 

 walls of the apparatus allowed every thing to be observed ; 

 and the glass-disc was less costly than one of brass would 

 have been. But on trying an experiment with the apparatus 

 thus arranged, the disc was found unable to withstand the 

 pressure of the atmosphere, when covered with a large bell- 

 jar, within which a tolerably perfect vacuum was produced ; 

 the glass-plate shivered into fragments with the noise of an 

 explosion, involving the bell-jar in destruction also. I shew 

 the Society pieces of the disc, which certainly might have been 

 expected to withstand the pressure to which it was exposed. 



I now, accordingly, recommend in preference the arrange- 

 ment which I exhibit. In it, the wires, instead of passing 

 through the sole or plate of the air-pump, are carried through 

 the upper part of the bell-jar, which has a brass collar and 

 top to admit of their passage, and is furnished with screws 

 for the attachment of the battery wires. They are insulated, 

 by being imbedded in ivory. Inside the bell-jar, the wires 

 admit of connection with the ends of the electro-magnetic 

 coils, by means of screws such as are employed in voltaic 

 arrangements. In using this form of apparatus, after con- 

 necting the wires inside between the top of the bell-jar and 

 the ends of the electro-magnetic coils, one of the battery 

 wires is connected with one of the screws on the outside of 

 the cap of the bell-jar, and with the other contact is made, 

 by touching the head of the second screw, and broken by with- 

 drawing the wire. Whenever contact is made the bell rings. 



Should it seem desirable to make the bell ring itself, it 

 could easily be done by placing one of Mr Bain's electric 

 clock pendulums between the battery and the bell. As that 

 ingenious automatic instrument alternately cuts off and lets 

 on (so to speak) the electric current, it would sound the bell 

 at each stroke of the pendulum. 



