332 Kreider— Experiments in Elementary Physics. 



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Fig. 2. 



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To illustrate the Grow cell, sis a type of the oxidizing depo- 

 larizers, the copper sulphate is withdrawn and washed out with 

 a little water — all of which proves of interest to the observer. 

 The platinum cathode is then inserted in place of the copper 

 and some concentrated nitric acid is ponred into B; where- 

 upon the galvanometer deflection is still greater and again 



remains constant. It is interesting to 

 show that both in the Daniell and in 

 the Grove cell the voltmeter instantly 

 returns to its original reading when 

 the galvanometer circuit is opened. If 

 tbe action goes on long enough in the 

 Grove cell, bubbles again appear : but 

 the difference from hydrogen is obvi- 

 ous both from the lack of the large 

 back EMF and from chemical tests 

 that may be applied. 



A similar cell, without the porous 

 partition, but with corresponding inter- 

 changeable electrodes, of which each 

 has a platinum, a lead and a copper 

 terminal, serves to illustrate a variety 

 of electrolytic effects, including the 

 principle of the storage cell. These 

 electrodes may of course be made with 

 four terminals each, instead of two, 

 to answer for both purposes. But it 

 is rather more satisfactory to have 

 two sets of electrodes to tit the same 

 holders, H. 



2. Boyle's Law Apparatus. — For 

 lecture room demonstrations the old 

 shepherd's-crook apparatus is of little 

 more value than a blackboard demon- 

 stration. Several features of the fol- 

 lowing piece of apparatus have made 

 its use here most satisfactory. The 

 manipulation is so simple and obvious 

 and the results so direct as to enable 

 each student, even those in the more 

 remote parts of the room, to make his 

 own readings and verifications of the law. So little time is 

 required for the manipulation that the number of experiments 

 may be multiplied and extended over different ranges, which 

 affords the student a more satisfactory demonstration and simul- 

 taneously an excellent mental drill. 



The volume tube is about 25 cm long, of uniform internal 



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