3-028-811 - The Print Collector/Heritage Images

Professor Tyndall lecturing at the Royal Institution [in London], 1870. Professor Tyndall,...after explaining the apparatus termed a " contact- breaker," employed in connecting and disconnecting the helix with the battery, and thereby making and breaking the circuit, which he elucidated by illustrations, exhibited the principal phenomena of the magnetic force. This force, which is really distributed through the mass of a magnet, appears to be concentrated at two points near the ends, termed poles, endowed with two opposite kinds of magnetism, one end termed north and the other south...In this doubleness consists magnetic polarity. The central portion of a steel bar-magnet was shown to be neutral, but when the magnet was broken into pieces every piece was shown to be endowed with polarity, a property residing in every particle. This property was also shown to be possessed by the copper wire constituting the helix of an electro-magnet, a suspended helix behaving like a magnetic needle. This magnetic power is distributed through the mass of the earth, with similar concentration at the poles. Professor Tyndall then demonstrated that by varying the power of the helix we vary the strength of the iron core within it'. From "Illustrated London News", 1870.