RECENT PROGRESS IN PHYSICS. 409 



If, for instance, a platinum wire of 1.5 metre in length and 0.5 

 millimetre in diameter is to be heated to redness^ how many Bunsen's 

 cups, of the electro motive force of 800 and the resistance of 10, must 

 be used and how are they to be combined? 



The resistance to conduction of a copper wire 1 millimetre in diameter 

 and 1.5 metre in length is 1.5; thatof alike platinum wire is 5.1,5 =:*7, 5. 

 But the resistance of a wire of one-half the diameter is four times as 

 great, viz : 30. This would be the resistance at the usual tempera- 

 ture ; but when the wire is red hot it is at least twice as great, viz : 

 60. If we suppose that the resistance of the other part of the closing 

 circuit is comparatively so little that it may be neglected, we have 

 r = 60, and, for our case, a = 172.0,5 = 86. ThereforCj 



nMO 



12.9 

 m 



= 86, consequently, n = 12,9 

 10 = 60, consequently, m = 2,1 ; 



from which it follows that a battery of 12 double elements has to be 

 employed. 



It is evident from this example that in the above mentioned exper- 

 iments the arrangement was not the most advantageous. 



If a copper wire 1 millimetre in diameter and 0.5 metre in length is to 

 be heated to redness, its resistance would be 1, supposing it to be twice 

 as great at a red heat as it is at the usual temperature. If the resist- 

 ance of the rest of the closing circuit is also equal to 1, its total will 

 be equal to 2 ; but a in this case is 433, and therefore 



n = 2.16, m = 10.8. 



We have, therefore, to use a battery of two elements, each of which 

 consists of 11 cups. 



A more accurate knowledge of the resistance to conduction of metals 

 at a red heat would be necessary to give a greater degree of exactness 

 to these calculations. 



In general more cups in a series will be required for producing igni- 

 tion if the wires are bad conductors and of greater length,, and more 

 cups, side by side in each element, if they are good conductors and of 

 greater diameter. 



§ 60. Ignition of metallic wires in different gases. — Grove has made the 

 remarkable observation that platinum wire heated to redness by the 

 voltaic current in atmospheric air, is apparently extinguished when 

 covered with a bell-glass, filled with hydrogen. — (Phil. Transact., 

 1847, pt. 1 ; Fog. Ann., LXXI, 196.) Since the resistance to con- 

 duction is greater in a wire intensely ignited than in one the heat of 

 which is less intense, it was to be expected that, cceteris paribus, the 

 same wire when in hydrogen would conduct a stronger current than in 

 atmospheric air. 



Grove proved the correctness of this conclusion in the following 



