Kreider — Experiments in Elementary Physics. 331 



Very satisfactory results may be obtained with any galva- 

 nometer wliose indications can be read by the class. Originally 

 we used one of tbe Max Kohl type, shunted with a variable 

 but low resistance, by means of which adjustment could be 

 made for a suitable range of deflections for the different cells, 

 without reducing the current through the cell to a magnitude 

 which would be unfavorable to the development of the phe- 

 nomena of polarization. A voltmeter connected in parallel 

 with the galvanometer circuit, while not essential to the 

 experiment, makes it much more complete. For the past two 

 years we have had the advantage of an admirable wall volt- 

 meter, which has a large illuminated scale, neatly executed 

 after our own design by the AVeston Instrument Company. It 

 is easily legible from all parts of the lecture room ; and dis- 

 tinctly shows the constant voltage of the Daniell and Grove 

 types of cells by the instantaneous return to the original read- 

 ing on open circuit, as well as a pronounced effect of mechan- 

 ical depolarization in the simple cell. 



With both of the compartments of the cell, A and B, filled 

 with dilute sulphuric acid and using the two copper electrodes, 

 the class observes that there is neither current nor difference 

 of potential. Changing the anode from copper to the amalga- 

 mated zinc results in a pressure of about 08 of a volt, and on 

 closing the galvanometer circuit a considerable current is indi- 

 cated. The rapid decrease of the deflection, with the simul- 

 taneous accumulation of hydrogen on the cathode, both being 

 visible to the class, clearly shows the cause and the effect of 

 polarization. On opening the galvanometer circuit the volt- 

 age is found to be lower than the original. Flicking the 

 cathode with one's finger sends the bubbles of gas scurrying 

 through the liquid, which is accompanied by an increased 

 deflection, illustrating a mechanical depolarization. The latter 

 effect is the more noticeable on the voltmeter with the galva- 

 nometer circuit open. 



After drawing off the contents of B, that compartment is 

 refilled with concentrated copper sulphate solution, which is 

 conveniently accomplished by the aid of a small flask with a 

 long neck. The Daniell cell thus formed shows the unit volt- 

 age and the galvanometer indicates an appreciably greater and 

 constant current. No gas appears ; but the class plainly sees a 

 change in the vicinity of the cathode, made obvious by absorp- 

 tion and refraction effects, which is evidence of the action of 

 which he has previously had an explanation.* 



* Oui' general plan has been to have the discussions of principles, includ- 

 ing blackboard sketches, etc., precede the experimental demonstrations 

 which are distributed through all of the exercises of the week. The arrange- 

 ment may appear wasteful of both time and energy ; on the other hand, it 

 has the great advantage of presenting the experiment at the moment when 

 it is most intelligible and significant. 



