404 



RECENT PROGRESS IN PHYSICS. 



Casselmann used a wire-holder similar to that represented in fig 4 J 

 By observing attentively a wire held by it while it is red hot, v, 

 perceive that in the immediate vicinity of the clamps its glow is coi 

 siderably less than in the middle. If now the wire be so far shorlene 

 that the cooling influence of the clamps extends to its middle it seem 

 easy to explain how, by shortening the length of the wire, the ph(i 

 nomena of ignition finally disappear. This is also seen from th 

 following observation : 



A platinum wire 0.21 metre in diameter was inserted in the circui 

 of a sinojle carbon-zinc cup. With a length of 3 centimetres it bt 

 came feebly red hot, while the tangent compass indicated 26°; bt 

 when the same wire was shortened to 1 centimetre no ignition wav 

 produced, even with a current of 34°. 



When, instead of the single element, two Bunsen's cups were used? 

 the appearances of ignition were entirely identical with the length; 

 of both 3 and 1 centimetre, though the corresponding deflection i 

 the former case was 34°, and in the latter (the shorter wire) 44°. 



§ 57. Relation beliveen the diameter and force of current in metallv 

 wires ignited by the galvanic current, — The above experiments do nc 

 illustrate the relation between the force of current and the diametc 

 of the wires, as corresponding to a certain degree of ignition, becaus 

 only the length, but not the diameter of the wire, was varied. 



The following table gives the results of a set of experiments mad' 

 with platinum wires of 1 decimetre in length and variable diameters' 



mm. 

 C.3 



0.39 



0.45* 



0.75 



Degree of ignition. 



Feeble 



Red hot 



Bright red 



Very bright red. 



Feeble 



Red hot 



Bright red 



Feeble 



Red hot 



Bright red 



Nearly white hot 



Red hot 



Bright red 



D 



163.9 

 169.4 

 1H2.2 

 210.0 

 163.7 

 1^^5.5 

 199.5 

 106.6 

 172.2 

 1-7.6 

 230. 3 

 161.7 

 209. 3 



i 



The experiments marked * are taken from the former series, (6(; 

 page 420.) _ . . , 



Jb'rom this series of experiments we may assume that, in order t 

 pi'oduce ihe same degree oj ignition^ the force of current must increaa 

 ^proportionally to the diameter of the ivires. According to this law, fo! 



