by which a Galvanic Luminous Arc can be produced. 443 



tually independent of the current, it is impossible to obtain a 

 luminous arc when the electromotive force (E) of the battery is 

 so much diminished that E is equal to D ; the sum of the elec- 

 tromotive forces of the entire circuit is then zero, and hence the 

 current must cease. But even before E, in being diminished, 

 attains these limits, the impossibility of forming a luminous arc 

 must occur ; for a certain intensity is necessary for its forma- 

 tion, the magnitude of which depends partially on other cir- 

 cumstances — as, for instance, the shape of the poles, the length 

 of the arc, &c. If, now, it should appear that a luminous arc 

 may be produced when E is as small as, or smaller than that value 

 of J) which is obtained in experiments with strong currents, this 

 must be regarded as a certain proof that in feebler currents D is 

 independent of their magnitude. 



3. It is the characteristic of the voltaic arc that material 

 particles are carried by the current from one pole to the other, 

 and that conduction is effected mainly by this transference. 

 It is often difficult to determine with certainty by the external 

 appearance whether there is a true luminous arc or not. When 

 there is an actual though imperfect contact between the poles, 

 the ignition and the combustion which generally follows the 

 former may be so great when the experiment is made in the 

 open air, that the phenomenon has the same appearance as 

 if there were a true voltaic arc. Hence it is easy to make a 

 mistake in this respect, if the external appearance only be taken 

 into consideration. In order to avoid such errors, a key with a 

 shunt was interposed in one of the wires leading from the bat- 

 tery to the place where the luminous arc was to be formed. In 

 some experiments there was a tangent-, and in the others a sine- 

 compass in the shunt. In one position of the key the principal 

 circuit was closed and the secondary one open ; in the other it 

 was just the reverse; and in the latter case the current could tra- 

 verse the secondary circuit, avoiding the break in the principal 

 circuit. When the light between the poles was really so bright 

 as to resemble an actual arc, the key was brought from the first 

 into the second position. During this the conduction was in- 

 terrupted long enough for the luminous arc to be extinguished. 

 If the poles were in contact, the magnetic needle made a deflec- 

 tion after the reversal, and the poles again began to be ignited ; 

 but if the luminous phenomenon was produced by a true voltaic 

 arc, the magnetic needle remained at rest. This mode of inves- 

 tigating whether there was a true voltaic arc or not, was of 

 course only needed when the intensity of the current was small, 

 and the voltaic arc so short that it could not be directly seen 

 whether the points were apart or not. 



