404) Prof. Dr. Jacobi on the Galvanic Spark. 



bably the cause also that at the opening of the circuit frequently 

 no spark is produced, when on making contact it had taken 

 place. On making contact, the external metallic surfaces melt 

 or soften together to a certain extent. By an adequate force 

 of the current it requires more revolutions of the micrometer 

 screw to separate them than was necessary to bring them into 

 contact ; we can also perceive under the microscope particles 

 which, as it were, are drawn out in the form of a wire. As 

 soon as the dimensions of the particles answer to the heating 

 power of the pile, they burn with the usual phaenomena 

 of combustion. We can therefore easily imagine conditions 

 in which all these phaenomena are absent; for instance, when 

 the current is weak, or the surfaces of contact large and power- 

 ful conductors of heat. Sometimes we also perceive two sepa- 

 ration sparks, which probably are produced by two places of 

 contact separated one after the other. 



The phaenomenon may now also be explained, that the se- 

 paration spark appears more intense by the use of an electro- 

 magnet, or of an electro-magnetic spiral. For here the heat- 

 ing effects of the electro-magnetic and magneto-electric cur- 

 rents combine. The disappearance of the magnetism by 

 which, as is well known, a magneto-electric current is pro- 

 duced, takes place, disregarding other circumstances inherent 

 in the qualities of the iron, for this reason, not instantaneously, 

 because contact can never be suddenly broken. For, the 

 more the surfaces of contact are pressed closely together, 

 forming what is usually termed perfect contact, the more do 

 they also increase in extension, or the smaller is the opposi- 

 tion to conduction. If we break the contact, this, properly 

 speaking, is merely a gradual diminution of the surfaces of 

 contact. By this consequently the opposition to conduction 

 is increased; with this decreases contemporaneously the force 

 of the galvanic current and of the electro-magnetism. The 

 magneto-electric current takes place in the surrounding spiral, 

 which however increases in intensity inversely to the decrease 

 of the original magnetism, and is exalted until its heating 

 power corresponds to the magnitude and other qualities ot 

 the exterior metallic surfaces of contact. Then originates 

 the process of combustion, which, however, now also causes 

 the complete separation of the surfaces of contact, and thus 

 entirely interrupts the magneto-electrical circle. The com- 

 bustion owing to the galvanic current takes place, we have 

 seen, on perfecting the contact, only when the points are 

 fine, or the surfaces of contact small ; in this case however the 

 current also becomes feeble, and the magnetism developed in 



