RECENT PROGRESS IN PHYSICS. 453 



§ 53. Phenomena following ignition.' — If the force of the current 

 is increased more than is necessary for the first incandescence the fol- 

 lowing phenomena appear in succession with the increasing force. 

 The wire becomes white hot, tears from its fastenings, breaks into 

 pieces, melts, and is dissipated. 



1. Tearing loose. — A platinum wire of 0.026 line radius and 16 

 lines long presented the following phenomena : 



According to Cavallo, the glow should progress from the positive 

 to the negative end of the wire. Biess noticed, with one exception, a 

 reverse progress in every case. 



According to Van Marum, when the wire is partially destroyed, it 

 is always the part nearest the positive coating that is injured ; but 

 Biess found the wire broken sometimes at the positive end and some- 

 times at the negative end. 



A wire which has once been brought partly to ignition, is more 

 easily torn than a new one. 



2. Breaking into pieces. — Wire being subjected to a stronger dis- 

 charge than is necessary to tear them, they break into a greater or 

 less number of small pieces giving oif light, which are thrown to 

 some distance. It may be seen in the collected pieces, that the dis- 

 membering of the wire depends upon a splitting and breaking action, 

 and that fusion where it appears is only secondary. 



A platinum wire 16 lines long, and 0.079 line thick, was sur- 

 rounded by a glass tube ^\ lines in diameter, and placed in the con- 

 ducting circuit. The discharge of the quantity of electricity, 22, 

 collected in 7 jars, brought it to ignition ; the quantity 35, tore it 

 into pieces, which were found in the tube. The pieces had evident 

 signs of fusion on their surface, and four of the largest seemed welded 

 together in a twisted figure, which indicated that they were thrown 

 while hot against each other, and against the sides of the tube. The 

 ends of all the pieces were not fused, most of them were sharp pointed. 

 A tolerably straight piece was measured under the microscope : it 

 was 0.081 line in the middle, and at one end 0.022 line in diameter, 

 hence it had been split lengthwise. Other pieces showed the same 

 appearance. 



Numerous other experiments gave similar results. By carefully 

 increasing the charge, the shivering of the wire was produced with- 

 out the least trace of fusion. 



3. Fusion. — By continually increasing discharges, the wires were 

 broken into less and less pieces, which melted at their surfaces and 

 ends, and at last flowed together, into globules. The wires were in 

 all cases torn violentlyfrom their fastenings, and the pieces scattered 



