256 Prof. Callan on a Contact Breaker 



About two years ago M. Foucault described in tlie Comptes 

 Rendus a contact breaker, in which the contact was made and 

 broken by a platina point caused to dip into mercury and rise 

 from it by means of an electro-magnet ; the mercury was covered 

 with alcohol. Soon after I had seen the description of this in- 

 strument, I made one which acted very well as long as I used 

 only one voltaic cell ; but when three cells were employed, the 

 alcohol took fire. I mentioned this circumstance to M. Foucault 

 at the meeting of the British Association in Dublin. He said 

 that he used thirty cells without inflaming the alcohol ; that in 

 bis contact breaker the alcohol over the mercury was about 2 inches 

 deep, and that in mine the depth of the alcohol must have been 

 too small. It was only about half an inch. 



In an instrument of this kind, I have lately used platina, 

 copper, amalgamated copper, silvered copper, tin, and iron for 

 making and breaking contact by dipping into mercury and rising 

 from it. The mercury was covered with oil, alcohol, or tur- 

 pentine. The same battery and electro-magnet were employed 

 to make these various metals vibrate rapidly. The amalgamated 

 copper acted with far greater effect than platina or any of the 

 other metals, but the contact was not made and broken so 

 rapidly as when any of the other metals was used, because a 

 small portion of mercury adhered to the amalgamated copper 

 after it rose above the surface of the mercury. Hence, in order 

 to break contact, it was necessary to make the amalgamated cop- 

 per and the armature of the electro-magnet vibrate through a 

 greater distance than that through which the other metals re- 

 quired to move. AVhen the amalgamated copper was used, it 

 was necessary to allow the armature to recede nearly -ik of an 

 inch from the poles of the electro-magnet. Hence the contact 

 was not made and broken with great rapidity. This defect 

 I have remedied by the use of a lever, the two arms of which 

 are at right angles to each other, and one of which is about 

 half an inch and the other 6 or 7 inches long. The short 

 ai'm is attached to the armature by means of a double-jointed 

 rod of brass or iron ; and the long arm, which must be horizontal, 

 to the amalgamated piece of copper. The electro-magnet is 

 fixed to a frame in a horizontal position ; the armature is screwed 

 to a board moveable on a hinge, and standing nearly at right 

 angles to the horizon. It is brought by means of a screw to the 

 distance from the poles of the electro-magnet which is found to 

 answer best. It is also pressed by a spring, or two springs, with 

 a force just sufficient to prevent the attraction of the electro- 

 magnet from drawing it to the magnet, and thus breaking con- 

 nexion with the battery before the magnet and core of the induc- 

 tion coil receive the full magnetic power which the battery is 



