in the (MiiTeiit, and if tliei'en|)()n the cnn-ent coinieetion is openecl 

 this is accompanied l)v a throw of the ballistic galvanometer in the 

 circuit of which the current is instantly extinguished and by a return 

 of the needle of the compass into the position which it would also 

 take up, if the curi*ent in the closed coil had been generated by a 

 magnetic tield equal to that of the current itself. The continuation 

 of the movements in • Maxwell's mechanism, an hen it has a supi-a- 

 conductor as carrier, is demonstrated by this experiment with ecjual 

 clearness and simplicity. 



§ 12. Comhinatlon of pai'idlel currents into one of <ireaf.er strength. 

 In trying to make the same experiments with mercury that we have 

 made with lead, it will be necessary in so far to change the experiment, 

 that one winding will be sufticient. This might be got by freezing 

 mercury in a capillary tube returning in itself with an expansion 

 head (like our other U-shaped mercury i-esistances). The chief 

 questions then are 1) if with a conductor of as large a section (keeping 

 for the present to the circular form) as would be necessary, with a 

 view to the threshold value of current density, in order to get an 

 action comparable to that with the lead coil, the thi'eshold value of 

 current density — of which as in N". 133 it is assumed that it is 

 determined princii)ally by the cui-rcnt density — does not undergo 

 a considerable diminution in consequence of the larger section, as 

 some considerations in N". 133 would make us fear, and 2) if we 

 can reckon with the mici-oresidual i-esistance as an ordinary resist- 

 ance even for such a completely d liferent section as that for which 

 it has been determined. An inducement to try the experiment imme- 

 diately with a lead ling') was a remark l)y my colleague EnRKNi'KST, 



1) I am glad to mention here that Mr. J. J. Taudin Ghabot of Degeiloch 

 (Würltemberg) shortly after my paper on the disappearance of resistance in mer- 

 cury and, as I found afterwards, only acquainted with my result, that the réi>ist- 

 ance of gold and platinum in an absolutely pure condition would probably disappear 

 altogether at extremely low temperatures, communicated to me a number of suggest- 

 ions regarding the condition into which meta's pass below this temperature and 

 which he would like to be considered as a distinct "fifth'" state of aggregation. 



Amongst these suggestions was the following : ''if a ring (of gold) is brought 

 to the condition of absolutely no resistance (in helium), an impulse (viz. by in- 

 duction) will be sufficient to produce a permanent current, which will make the 

 ring into a magnetic shell, as loiig as the temperature of the metal remains below 

 a certain critical value". By critical value was meant — not the vanishing point 

 as discovered afterwards — but the temperature charactei-islic of each metal at 

 which, according to my earlier views, the resistance of the pure metal would 

 become zero independently of the current-strength. The idea, however, undeilying 

 this speculation — which was further developed by supposing the cooled ring to 



