Intelligence and Miscellaneous Articles. 451 



is required. The ordinary carbon megohm is very untrustworthy, 

 being subject to considerable variation and unsuited for use with 

 pressures of more than a few volts. The Author having overcome 

 these defects thinks the simplicity of the plan followed no reason 

 for withholding a description of it. 



A study of some of these carbon megohms, supplied by 

 instrument-makers, showed that, although the loss is very small, 

 being only one-millionth of a watt per volt impressed, the delicate 

 conducting film or line of plumbago is too much disturbed by the 

 heat generated. Experiment showed that on increasing the cross- 

 section of the conducting film, and correspondingly increasing its 

 length, it became easy to construct a carbon resistance practically 

 unaffected by any ordinary variation of temperature, and capable 

 of being jised with pressures of 100 volts per megohm, or as much 

 higher as may be desired. 



Various ways of carrying this out have been tried. Excellent 

 results were obtained by the use of long strips of shellaced 

 cartridge-paper coated on one side to a width of one to two 

 inches with plumbago, well burnished, the edges of the paper 

 strip being folded down over the film, to protect it. The strip 

 is then rolled into a loose spiral, secured to a support, and mounted 

 in a suitable box with terminals. 



Examples of these resistances were shown and particulars given 

 of tests to which they had been subjected. The resistance in one 

 case was 1-315 megohm at 74° F. It was tested at various 

 temperatures up to 150° F., and was practically constant throughout 

 this range, the resistance at the higher temperature being 1*31 

 megohms. 



Another resistance shown, which measured 0'975 megohm, had 

 been subjected to a pressure of 100 volts for 12 hours continuously 

 without showing any change. It was stated that these resistances 

 were being supplied by Mr. Paul, of 44 Hatton Garden, London. 



SEARCH FOR SOLAR ^-RAYS ON PIKERS PEAK. 

 BY FLORIAN CAJORI. 



Experiments carried on by M. C. Lea * and others have failed 

 to show the presence of Eontgen rays in solar radiation. If 

 these rays reach us from the sun, their intensity must be ex- 

 ceedingly feeble. The suggestion has been made that Eontgen 

 rays may exist in sunlight, but are absorbed by the earth's atmo- 

 sphere. The fact that Lenard rays are stopped by only a thin 

 layer of air made it not improbable that Eontgen rays might 

 be stopped by a thick layer of it. On this hypothesis a mountain- 

 top is the best locality to examine sunlight for the new rays. The 

 writer determined, moreover, to expose the photographic plate to 

 solar rays, not several hours, but several weeks. 



* Am. Journ. Sci. [4] i. 189G, pp. 363, 304. 



