574 



NATURE 



[April 14, 1892 



Physical Society, March 25.— Prof. S. P. Thompson, Vice- 

 Presiiient, in the chair. — A note on the electromotive forces 

 of gold and platinum cells was read by Prof. E. F, Herroun. 

 Modern text-books put gold before platinum in Vulta's eleciro- 

 positive series, and thus one is led to expect a greater evolution 

 of heat when gold combines with (say) chlorine, than when 

 platinum does so. This, however, is not the case, for Julius 

 Thomsen gives for the heat of formation of platinic chloride a 

 value considerably greater than that for auric chloride. Gold 

 should therefore be electro-negative to platinum. The few ex- 

 perimenters who have tested such cells, arrived at different con- 

 clusions, hence the author took up the subject, and examined 

 experimentally the E.M.F.'s of zinc-platinum and zinc-gold 

 cells, the metals being immersed in solutions of their chlo-rides 

 of equal molecular strength. Instead of platinic chloride a solu- 

 tion of sodio platinic chloride was employed. From Thomsen's 

 thermo-chemical data, the E.M.F. of such a zinc-platinum cell 

 should be i'548 volis, whilst experiment gave values between 

 170 and I '473, according to the previous history of the cell. 

 The average E.M.F. was about i"S25. Allowing the cell to 

 . send a current reduced the E.M.F. considerably, hut it partly 

 recovered on standing. Renewing the sodio-platinic chloride 

 reproduced the high initial E.M.F. of 17 volts. This high 

 value, and the uncertainty of the E. M. F. after sending a current, 

 the author believed due to dissolved oxygen. Zinc-gold cells, 

 the metals being immersed in solutions of their chlorides, gave 

 more constant results, the maximum being I '855, and the mini- 

 mum I "834 volts, whereas from thermo-chemical data the E.M.F. 

 should be 2 '044. On replacing a gold plate by a platinum one, 

 the E.M.F. fell to 1782. Other experiments showed that gold 

 is slightly electro-positive to platinum in water or dilute HCl, 

 but in aqua regia the positions are reversed. Prof. Ayrton said 

 the expi-rimental E.M.F.'s were fairly close to the theoretical 

 values, and thought the differences might arise from occlusion of 

 gases, which, although not taken into account in the thermo- 

 chemical experiments, might have considerable effect on the 

 electrical values. Platinum, especially, had remarkable oc- 

 cluding properties. Mr. Enright* pointed out that, if any gases 

 were disengaged by the reactions in the cells, their thermal 

 values must be allowed for. The Chairman (Dr. Thompson) 

 believed that some discrepancy between the calculated and ob- 

 served values of the E.M.F.'s might be due to the calculations 

 only being carried to the first degree of approximation. The 

 complete expression contained, amongst others, a term depend- 

 ing on the temperature coefficient of the cell. On the subject of 

 variation of the sign of E.M.F. with the strength of solutions, 

 he said he had observed similar effects with cyanide solution. 

 Dr. Herroun, in reply, said care was taken to expel as 

 much of the occluded gas as possible before using the 

 plates, and no gases were formed in the reactions. To 

 Dr. Thompson he pointed out that Clark's cell had an E.M.F. 

 greater than that calculated from thermo-chemical data, hence 

 the temperature coefficient ought to be positive, but, as a matter 

 of fact, it is negative. The discrepancy between the calculated 

 E.M.F. and the observed he believed due to inaccurate deter- 

 minations of the thermo-chemical constants of mercury salts. — 

 A new instrument for showing the effects of persistence of 

 vision was exhibited and described by Mr. E. Stuart Bruce. 

 The instrument, which the author calls an " aerial graphoscope," 

 consists of a narrow wooden lath mounted on a whirling machine, 

 so as to be rotated rapidly in its own plane. The lath is tinted 

 gray in the centre, and shades off to white at the ends. When 

 rotated rapidly, it presents the appearance of a nearly uniform 

 screen or disk, owing to persistence of impression. Ordinary 

 lantern-slides were projected on this aerial screen with remark- 

 able effect, for the pictures appeared suspended in mid-air. The 

 author explained that the object of darkening the lath near the 

 middle was to give a more uniform illumination to the picture 

 or disk. On covering up the centre portion of the lath with 

 ■white paper, the middle of a picture projected on it was much 

 more strongly illuminated than the ed^^es. Mr. Blakesley 

 pointed out that the effect produced by darkening the centre of 

 the lath might be attained by painting white sectors on a black 

 lath. — A papier on some electrical instruments was read by 

 Mr. R. W. Paul, and the apparatus exhibited. He first de- 

 scribed a new form of standard ohm, the distinguishing feature 

 of which is that the wire is wound in one flat spiral, and con- 

 tained between two thin brass plates. The whole of the wire 

 is thus practically at the same level in the water-bath, and 

 therefore will be more likely to be at unifirm temperature 

 throughout than coils having considerable vertical depth. A 



NO. II 72, VOL. 45] 



thermometer passing down the central tube has its bulb on the 

 same level as the wire ; and another thermometer, placed in the 

 water-bath at the same level, serves to check the uniformity of 

 temperature. In order that the width of the coil may not pre- 

 vent convection currents in ihr bath, the screws which fix the 

 two brass plates together have large holes through them. Dr. 

 Fleming's suggestion of forming the upper ebonite insulator 

 into an oil cup has also been carried out. A new form of 

 Wheatstone bridge was next shown, possessing all the advant- 

 ages of the dial pattern combined with great facilities for clean- 

 ing. There are four resistances in each proportional arm, and 

 the adjustable arm has four sets of coils — units, tens, hundreds, 

 and thousands — each set consisting of ten equal coils. The 

 ends of each coil are connected to bra s sockets, fixed, about an 

 inch apart, on the ebonite top. Successive coils are put in 

 circuit by placing a plug attached to a Hexible cord in the re- 

 quired socket. .Special contact-bars are provided, whereby two 

 or more coils of any set of ten may be put in parallel arc, 

 so as to get accurate resistances of large carrying capacity. 

 These bars are also useful for obtaining high ratios between two 

 resistances, a point of considerable importance in the testing of 

 large resistances. Amongst the advantages claimed are: better 

 insulation, avoidance of surface leakage tiy providing ample 

 facilities for cleaning, small block error which is constant and 

 easily measured, and no loose plugs required. Each set of ten 

 coils may be used as separate circuits. By means of two tra- 

 vtlling terminals the box may also be used as a potentiometer 

 reading to I part in 10,000. A reflecting galvanometer with 

 several improvements was then exhioited and described. The 

 coil is supported on an ebonite pillar fixed to a tripod, below 

 the centre of which controlling magnets on the Siemens 

 principle are pivoted. The pillar gives good insulation from 

 earth, and the adjustment of the control can be made without 

 setting the needle in vibration. The two halves of the coil are 

 wound according to Sir W. Thomson's law, and fixed in 

 ebonite boxes turned to fit them. They are thus kept perman- 

 ently in shape. The ebonite boxes are interchangeable, so that 

 either high- or low-resistance coils can be used 'n the same 

 stand. The coils have separate terminals, and ^.an therefore be 

 used in series or parallel or differentially. The mirror is placed 

 in a metal box below the coils. When intended for an astatic 

 instrument, magnets are put behind the mirror, and the metal 

 box serves to damp the vibrations. For ballistic work the mirror 

 has no magnets on it, and the damping may be regulated by 

 sliding in or out a plug which carries the window of the mirror 

 box. Mr. Swinburne inquired whether the plan of using two 

 vertical magnets to form an astatic system had been tried, and 

 with what result. He also asked if dial bridges made with 

 switches instead of plugs would not be advantag. ous. Dr. 

 Sumpner said vertical needles had been used at the Central Insti- 

 tution, and found satisfactory. Mr. A. P. Trotter wished to 

 know whether there was any very great advantage in designing 

 galvanometers with a minimum amount of wire. A galvano- 

 meter was often required for many different purposes, and it did 

 not follow that one with a minimum amount of wire was the 

 best all-round instrument. Mr. C. W. S. Crawley made in- 

 quiries as to the magnitude of the block error in the form of 

 Wheatstone bridge shown, for he thought the flexible cords 

 would make it considerable. In reply to Mr. Swinburne, he 

 said he had found the variations in switch bridges greater than 

 in plugs. Prof. S. P. Thompson thought it was not generally 

 known that the best shape of galvanometer coil depended on 

 whether the instrument was to be used as an ammeter or 

 voltmeter. The shape determined by Sir W. Thomson was 

 a voltmeter coil ; that for an amme'er was much shorter 

 axially. Mr. Paul, in reply, said he used one or other shape of 

 coil according to the use for which the galvanometer was 

 intended. The block error in the Wheatstone bridge was very 

 small, and quite negligible for most purposes. When very great 

 accuracy was required, the error, being constant, was easily 

 measured and allowed for. 



Royal Microscopical Society, March 16. — Dr. R. Braith- 

 waite. President, in the chair. — Mr. G. C. Karop exhibited and 

 described Messrs. Swift's new fine adjustment to th' substage. 

 Mr. Karop stated that in this su >stage one complete revolution 

 was equivalent to a vertical movemmt of the tss'^ "^ ^^^ inch. 

 — Mr. E. M. Nelson gave a rSsumS of the contents of two 

 papers, the first of which was entitled "Virtual Images and 

 Initial Magnifying Power," and the other 'On Penetration in 

 the Microscope." — Dr. W. H, Dallinger said that an important 



