. The two common failures of the Clark standard cell . u and elsewhere has beento seal the terminal wire through the cell wall in such a way asto leave all but the end of the wire within the cell covered by athin sheath of glass. This method of sealing in, aheady describedby Hulett,® was thought to render the cell less liable to crack,particularly if fine wire was used. However, with the smallestsize platinum wire practicable for use as terminals, the sheath ofglass has afforded little or no protection against cracking of theamalgam limb of zinc cells. 3. GAS FORMATION The formation of gas in
. The two common failures of the Clark standard cell . u and elsewhere has beento seal the terminal wire through the cell wall in such a way asto leave all but the end of the wire within the cell covered by athin sheath of glass. This method of sealing in, aheady describedby Hulett,® was thought to render the cell less liable to crack,particularly if fine wire was used. However, with the smallestsize platinum wire practicable for use as terminals, the sheath ofglass has afforded little or no protection against cracking of theamalgam limb of zinc cells. 3. GAS FORMATION The formation of gas in the amalgam limb of the zinc cell seemsto have been observed first by Schuster, ^^ since in constructinga large cell at Manchester in July, 1891, a mercury valve wasprovided to permit the escape of gas if evolved. Its causes were ^ Phil. Trans., 175, p. 443; 1884. 8 Phil. Trans., 176. p. 784; 1885. ^ Phys. Rev., 32, pp. 271-272; 1911. 10 Glazebrook and Skinner, Phil. Trans., 183, p. 588; 1892. McKelvy 1Shoemakerj Failures of Clark Standard Cells 413. ?AMALGAM considered first by Kahle/^ who observed that increased evolu-tion of gas occurs with free acid in the zinc sulphate solution; alsowith imptue zinc, particularly if it contains arsenic. In the lattercase the formation of gas is ascribed to local currents within theamalgam and is said to have occtirred when the purest of zinc saltwas employed. Where the pure zinc and pure neutral salt wereused, no evolution of gas hadbeen observed by Kahle atthe time of writing. Inas-much as this investigator*^and other investigators atthe Physikalisch-TechnischeReichsanstalt later mentiontrouble resulting from thistype of failure, it is safe toconclude that, with the pur-est materials they were ableto obtain, the difficulty was not entirely obviated. ^^^- ^ -—Form of Clark cell proposed by Jaeger Our experience shows that even with materials of unquestion-able purity the evolution of gas is often sufficient to cause liftingof the crysta
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