102 'SI. W. G. Haiikel on the Development of Electricity 



electx'ical tension had to be measured, either the crucible (its 

 support being isolated) or the internal wire was connected either 

 with the gold-leaf of the electrometer (see part 1 of my electrical 

 investigations, vol. v. of the Abhandlungen der Koniglichen Ge- 

 sellschaft Sachsens, where the electrometer is denoted by A), or 

 with one plate of a condenser, the wire or the crucible being 

 respectively in connexion with the earth. For the sake of brevity, 

 I shall call a current proceeding from the crucible to the wire 

 an ascending current, one from the wire to the crucible a de- 

 scending one. 



If the melted mass of the salt be allowed to cool after the 

 wire has been introduced, and be then heated again, at the com- 

 mencement of the heating an ascending current is formed in the 

 solid salt, if it conduct at all : the current passes from the hotter 

 to the cooler metal. 



As soon as the salt in contact with the sides of the crucible 

 begins to melt, cause is given for a descending current ; at the 

 same time, however, the temperature of the wire is generally 

 superior to that of the wire in the still solid mass of the salt. 

 The direction in which the needle of the galvanometer is deflected 

 in consequence of the two electrical forces, which act in exactly 

 opposite directions, that is, whether it be deflected in accord- 

 ance Avith an ascending or descending current, depends upon 

 the relative strengths of these two forces. In many cases, not 

 only the strength, but even also the direction of the deflection 

 may be changed at will by altering the relative magnitude of 

 these forces. 



After the fusion is complete, an ascending current is formed 

 (except in the case of the sulphate of potash and copper), as the 

 difi"erence of temperature would lead us to expect. 



On extinguishing the lamp before the salt begins to solidify, 

 the relation of the two metallic poles as to temperature is the 

 inverse of what it was during the application of the heat, and 

 consequently an ascending current will be formed. [Sulphate 

 of potash and copper, nitrate of potash and nitrate of soda are 

 exceptions to this rule; in these salts a descending current 

 occurs.] 



The subsequent solidification, whereby the side of the ci'ucible 

 is in contact with solid and the wire with liquid matter, gives rise 

 to an ascending current, that is, to one in an opposite direction to 

 that caused by the previous melting of the mass. Whether, 

 however, this cause is sufliciently ])owerful to determine the de- 

 flection of the needle in the corresponding direction, and hoiy 

 great such deflection may be, depends, as before, upon the rela- 

 tive magnitude of the electrical forces called into action on the 

 one hand by the difi\;rence of temperature (of the platinum- 



