670 EFFECTS OF THE GALVANIC CURRENT. 



same current, and one of the same thickness, but only 

 4 cm long, is fused and begins to volatilise. 



It has already been stated that the tension of current 

 electricity in an ordinary battery is too small to produce 

 sparks. Batteries of several thousands of elements 

 have been constructed, and by means of these a constant 

 series of sparks has been obtained, passing between 

 two terminals separated by a very small distance. 

 Sparks are nevertheless ordinarily seen between the 

 two terminals when contact is made and then again 

 broken; frequently a minute spark is seen when the 

 circuit is closed, but at any rate a spark appears when 

 it is opened. The appearance of the spark in this case 

 is due, not to incandescent air, but to incandescent 

 metal ; the two terminals touch each other only at a 

 metallic point which is sufficiently heated by the 

 current to become luminous and to be volatilised. 

 When the terminals consist of copper wire, the sparks 

 are very small and appear bluish-green ; iron wires 

 produce large scintillating sparks of a yellowish- red 

 colour. With copper wires, brilliant yellow-red sparks, 

 interspersed with tiny bluish-green luminous points, 

 may be obtained if one terminal be pressed upon the 

 uncut part of a smooth file near the handle, and the 

 other is drawn over the cut portion so as to touch only 

 the projecting points ; in this experiment the points of 

 the teeth of the file and the copper become incandescent 

 at the same time. 



When two pieces of some compact well-conducting 

 variety of carbon are attached to the ends of the con- 

 necting wires, and gentle contact is made between 

 them, a small but very bright luminous point appears 



