Dr Wilson on the Electrical Magnetic Bell. 311 



It consists of the ordinary electro-magnet, that is, of a 

 horse-shoe of soft iron, surrounded by coils of covered copper 

 wire, terminating in free extremities, which can be connected 

 with the electrodes of a voltaic battery. The horse-shoe is 

 fastened horizontally to a wooden stand, and between its 

 limbs, near their exposed poles, a vertical rod is placed, sup- 

 porting a time-piece bell. In front of the ends of the horse- 

 shoe, a horizontal bar of soft iron is hung upon vertical pivots, 

 so as to move through a small arc, towards or from the poles 

 of the magnet ; and to this bar or keeper a small hammer is 

 attached, which strikes the bell, when the keeper is attracted 

 to the magnet. 



When the coil-wires are connected with a battery, the 

 horse- shoe becomes a magnet, and pulls the keeper towards 

 it, which carries with it the hammer and strikes it on the 

 bell. When the battery connection is cut off, the keeper and 

 hammer fall back, and by alternately connecting and dis- 

 connecting the wire proceeding to one pole of the battery, 

 whilst the other remains in galvanic connection with the oppo- 

 site electrode, the bell may be rung as often as required, and 

 the strokes made to follow each other at whatever intervals 

 of time are desired. This is the arrangement at present in 

 use in our railway telegraphs, and is adopted without any 

 modification in the apparatus before us, for the construction 

 of which I am indebted to the kindness of Mr Bain. 



In order that this Electro-Magnetic Bell may be available 

 for experiments on the gases, it is necessary to have the means 

 of making and of breaking connection with the battery, whilst 

 the bell arrangement is under a glass-jar on the air-pump 

 plate. To secure this the wires must be brought air-tight, 

 and insulated through the sole of the pump, or through the 

 sides or top of the bell-jar, so as to admit of their connection 

 with the battery at a distance from the air-pump. In the first 

 arrangement I made use of for this purpose, I employed a 

 disc of plate-glass, afoot in diameter, of the thickness of quar- 

 ter an inch, and which was furnished in the centre with a 

 hollow brass-tube, open at both ends, and provided with a 

 screw, so as to enable it to fit into the central aperture of 

 the ordinary air-pump plate. Near the centre of this disc were 



