A iaAerfiresented at the General Meeting of theA merican Institute of Electrical Engineers,Chicago, ill.,June 8th, S892, President S-raguein the Chair.

THE REVERSAL OF POLARITY IN PLATINGDYNAMOS.

BY PROFESSOR HARRIS J. RYAN.

Shunt machines used for copper plating carbons are frequentlyfound to have their polarity reversed after being stopped. Thecause can be ascribed only to voltaic action of the unplated orpartially plated carbons, copper aniodes, and the plating solution.Any electromotive forces in the bath that would result from theelectrolytic action of the dynamo current must be counter to theE. M. F. of the dynamo. Any curreiit that these counter E. M. F.'smight set up through the dynaino when at rest, as is well known,could tend to only maintain the same polarity.To determine the extent of the voltaic action Messrs. Thayer

and Warner, of the Senior class in Electrical Engineering at Cor-nell, miade a copper plating bath of the density that is found to dobest for copper voltineter work. This solution was increased20 per cent. by the addition of commercial sulphuric acid so as toobtain a solution practically the same as that used by the carbonplaters for good tough deposits. In this bath an unplated elec-tric light carbon formed the cathode and good commercial copperthe anode. With this cell the observations given in the follow-ing table were made.The E. M. F.'5 of the cell due to voltaic action were observed by

the deflection of a galvanometer, connected to its terminals inmmne-diately after the circuit of the electrolytic currenlt was broken.The table explains itself, and shows that the difference of po-

tential between the unplated carboil and copper is .17 of a volt.After the deposit of a thin film of copper oni the carbon thisE. M. F., immediately after stopping the current, was .004 of a

RYAN OI REVERSAL OF DYNAAMOS.

volt, and was rising so that at the end of 10 minutes it was .138volts. When a inarked deposit was made by the applicationl ofthe electrolytic current for one minute the cell gave a counter E.M. F. of .085 of a volt immediately on stopping the current. Thereturn of the voltaic E.M.F. was evident, for two iminutes later theE. M. F. of tthe cell was zero, and sixteen minutes from the time ofstopping the current the voltaic E. M. F. of the cell was .130 of avolt. At this point the application of the current for but fiveseconds produced a momentary return of the counter E. M. F. ofthe cell. Even after the carbons had been well plated by the ap-plicationi of the electrolytic current for fifteen minutes, wlhen thecounter E. L. F. of the cell at the moment of breaking the circuit

Time. Volts, Remarks.

11:20 -.17o Carbon unplated.II:20/2 -.004 Current on 30 seconds ; lower part of carbon plated; plating11:28 -.012 on the upper part scarcely perceptible.11:30 - s3811:32 +.085 Current on i minute; well-marked deposit on carbon.1 I:34 .00011:36 -.0221I:40 -.06411:48 -.I301II:s0 +.o64 Current on 5 seconds.11:52 -.12412:15 +.138 Current on 13 minntes.I2:20 +.064I2:36 +.05312:55 +.0211:45 +.oo6

+ indicates a counter E. M. F.-indicates a voltaic E. M. F.

was .138 volts, there was a strong tendency manifested for thereturn of the voltaic E. M. F.

It is seen, therefore, that with a shunt eleetroplating machine atwork oIn a bath for plating carbons, there is danger of reversingits magnetism wlhenever it is stopped before the carbons are wellcoated. It mnay be avoided by breaking the main circuit beforeshutting dowin, and inakinig it again only after the maclhine hasbeen brought to full speed. This method would generally requirethe use of a large and expensive knife switch. Breaking andmaking the field circuit ill the same mannier would do just aswell.

[June 8,526