Neue Versuche zur Deutung der Feinwanderung in elektrischen Abhebekontakten
The well known bridge transfer ("Feinwanderung") - occurring even in non-arcing electric breaking contacts - is caused by the formation of bridges of molten metal between the separating electrodes. The hottest cross-section of the bridge seems nearly always to be displaced to the anode; the repeated rupture of such asymmetrical bridges forms a crater in the anode and a cone on the cathode. Only Pt contacts show a change of the direction of the transfer with increasing current. The inversion current strength can be shifted by changing the length and diameter of contacts; thus one can make contacts which are free of disturbances by transfer when used with currents between 10-20 amperes. The temperature asymmetry of the liquid bridge was explained by the superposition of the Thomson- and Peltiereffects over the Joulean heat. These effects have not been measured up to now but we have computed them approximately from the Thomson formulae using the measured Seebeck-effects in the case of Zn and Bi. The observed "positive" direction of material transfer (anode -> cathode) can not be satisfactorily explained by this theory. A supposition about the possible influence of the second Benedicks-effect was quite surprisingly confirmed in discussions with M. Kohler, who had formerly proved that this thermoelectric effect between identical electrodes is caused by their adsorbed gas layers. Thus the electron tunnelling through the potential wall of the contact space preheats the anode by its kinetic energy and supports a positive sign of bridge transfer. Experiments to avoid bridge transfer by use of higher melting anodes failed to be 100% successful; but the reduction of the work function of the cathode by oxidation diminishes the kinetic energy of the tunnelling electrons and seems to be successful in Au alloy contacts.
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