EECEST PSOGEESS IN PHYSICS. 419 



tlirougli a long circuit, including the fine igniting'wires, with the nega- 

 tive pole, so thatfB may be considered the negative pole and A the 

 positive. The current circulates and produces ignition in the iron 

 wire as soon as A and B come into contact. 



In order to pull the disk B towards A from a distance, two pieces of 

 twine fastened to B pass through holes in the disk A, and at E are 

 connected with the long string that reaches to the place whence the 

 person who is to close the circuit stands. An accidental discharge 

 is prevented by a peg F between A and B, which must be removed 

 before the two disks can come in contact. But besides lessening the 

 danger this method of blasting oifers other considerable advantages; 

 it enables us v/ithout much difficulty to explode powder under water. 

 For this purpose the entire charge is to be enclosed in a water-tight 

 tin box and this put in the place where its action is desired. 



The application of galvanic ignition is also very advantageous when 

 great masses of rock are to be blasted. Formerly, in such cases, it 

 was necessary to use a heavy charge in one great mine, but several 

 smaller properly distributed charges would produce a much greater 

 effect if they could be ignited simultaneously. This can now be done 

 by the aid of the galvanic current ; the connecting wires have only to 

 be so arranged that all the holes are at the same time in the circuit. 

 In this way immense effects have been obtained in England. 



What power the battery must have in each case can easily be ascer- 

 tained from preceding sections. From section 59 can be ascertained what 

 force of current is required to make the thin iron wire incandescent, 

 (the diameter of which must of course be known,) and after computing 

 the resistance in the conducting wires, it is easy to determine how 

 many cups or pairs of plates of any give point must be used and how 

 they must be arranged to produce this force of current. 



§ 64. The voltaic arc. — By the construction of the constant battery, 



! the production of the arc of light which Davy was the first to observe 



is greatly facilitated, and hence this interesting phenomenon has beeE 



several times investigated, though much is still left for furthei 



researches. 



De la Kive paid great attention to the galvanic arc; v;-e take tha 

 following from his elaborate treatise on this subject, published in 

 Phil. Trans., f. 1847, (Pogg. Ann., LXXVI, 170.) 



The voltaic arc can be produced not only between carbon points but 

 also between points of different metals. It is greater with the more 

 fusible or oxydisable metals, as zinc or iron, than with platinum or 

 silver. The size of the arc of light is proportioned to the greater or 

 less facility with which the substance of the electrode disintegrates ; 

 for since this phenomenon is produced by minute particles ot 

 matter carried over from one electrode to the other, its formation must 

 necessarily be favored by a less cohesion of the electrodes; this is also 

 the reason why, under otherwise like conditions, the greatest arc of 

 light is always obtained between carbon points. The transference of 

 the matter is always from the positive to the negative pole. In the 

 air and with metallic electrodes, the deposit upon the negative pole 



