672 THE ELECTRIC .LIGHT. 



ing medium between the poles, and the quantity of in- 

 candescent matter is thus much greater than it would 

 be if mere contact of points were maintained. 



For these experiments copper wires, not less than l mm thick, are 

 attached to the poles of the battery. The thin copper wire which 

 is to be heated may be simply pressed with the fingers to the free 

 ends of the terminals, or, still better, two screw clamps are used, to 

 which first the thin wire and then the stouter terminals are fixed. 

 Clamps must, of course, be used when iron wire is to be made red 

 hot or is to be fused, or the fingers will be burnt. The iron wire is 

 first fixed in the clamps at both ends ; one terminal is then screwed 

 into one clamp, and the other is pressed by the hand upon the second 

 clamp. The same piece of iron wire which has been rendered 

 incandescent by the current cannot be fused easily in the next 

 experiment unless it is previously made bright by polishing it with 

 emery paper ; the reason is that while it is red hot a thin layer of 

 oxide of iron is formed on its surface, which is a bad conductor of 

 the current. 



For obtaining the brilliant speck of light between two carbon 

 points (which with only two elements cannot give the least idea of 

 the splendour of the electric light), it is best to purchase one of the 

 small bars of ' gas graphite,' 15 or 20 cm long and a few millimetres 

 thick, which are expressly made for the electric light, and may be 

 had at the dealers in electrical apparatus. Two bits, each 4 or 5 cm 

 long, are cut off the bar, and copper wire is coiled round one end of 

 each of them, forming about eight or ten close turns, and leaving a 

 straight piece of the wire to project at the end, which serves for 

 connecting it with the wires from the battery by means of a binding 

 screw. The copper wire should first be softened over the flame and 

 then polished with emery-powder before coiling it round the carbon 

 pieces. The free ends of these may be made pointed by the file ; 

 but it is quite as well to leave them as they are and bring them 

 together by the edges, taking care to make gentle contact at one 

 point only. As soon as the least pressure is applied, so that the 

 surface of contact is increased and the current no longer passes 

 through minute projecting points, a dull glow is obtained instead 

 cf a brilliant point of light. 



If carbon points of the kind described cannot be obtained, two 

 oblong pieces of dense coke may be substituted. If Bunsen elements 

 are used, the copper wire from the zinc pole may be simply placed 

 in contact with the carbon plate which forms the opposite pole, and 

 a bright dot of light will appear at the point of contact. 



