In the opinion of Matthieson and of Klemencic the 10 per cent silver, 90 per cent platinum alloy is the one most suitable for resistance standards. At all events, it has stood the test of time, for, with the following exceptions, all the British Association coils constructed of it from 1867 to the present day have continued to agree well together. The exceptions were three one-ohm coils, which permanently increased between 1888 and 1890, probably through some straining when immersed in ice. One coil changed by 0.0006 in 1 between the years 1867 and 1891. According to Klemencic, absolute permanency is not to be expected even from this alloy. Its recommendation as a standard depends on its chemical inertness, its small temperature coefficient (0.00027 per degree), and its small thermo-voltage against copper, as the following table (taken from Klemencic) will show:- Thermo-voltages in Micro-volts per degree against Copper over the Range 0 deg. to 17 deg. C. Platinum iridium 7.14 micro-volts per degree C. Platinum silver 6.62 micro-volts per degree C. Nickelin 28.5 micro-volts per degree C. German silver 10.43 micro-volts per degree C. Manganin (St. Lindeck) 1.5 micro-volts per degree C. Mechanically, the platinum silver is weak, and is greatly affected as to its resistance by mechanical strains--in fact, Klemencic considers it the worst substance he examined from this point of view--a conclusion rather borne out by Mr. Glazebrook's experience with the British Association standards already referred to (B. A. Reports, 1891 and 1892). Taking everything into account, it will probably be well to construct standards either with oil insulation only, or to bake the coils in shellac before testing, instead of soaking in paraffin. Fig. 89 illustrates a form of an oil immersed standard which is in use in my laboratory, and through which a considerable current may be passed. The oil is stirred by means of a screw propeller. Fig. 89.