530 



SCIENCE 



[N. S. Vol. XXV. No. 640 



circuit with the galvanometer, ready to be 

 used for calibrating the latter at any time. 

 They can be made in the laboratory at 

 trifling cost and can be sent by mail or 

 express to the Bureau of Standards and 

 calibrated at very slight expense. Speci- 

 fications for such coils will shortly be pub- 

 lished by the bureau. 



The Mutual Inductance of Two Coaxial 



Coils: E. B. Rosa. 



Absolute measurements of resistance 

 have been made by Rowland, Glazebrook 

 and others, using two coaxial coils the 

 mutual inductance of which was computed 

 from their dimensions. By the methods 

 employed by Kohlrausch and Rayleigh the 

 mean radius of such coils can be obtaiaed 

 with great precision by comparing them 

 with larger coils wound with a single layer 

 of wire, the mean radius of which may be 

 determined with sufficient accuracy by 

 direct measurement. The formulee em- 

 ployed by Rowland and Glazebrook in cal- 

 culating the mutual inductances of their 

 coils are both approximate, and do not 

 agree with one another closely unless the 

 coils have very small sections or are rather 

 far apart. Weinstein and Stefan both 

 gave formulae for use in calculating the 

 mutual inductance of such coils, but they 

 do not agree with one another as accurately 

 as is desirable for precision work. I have 

 revised and corrected Weinstein 's formula 

 and derived a new formula starting where 

 Stefan began, these two formula agreeing 

 very closely and proving more accurate 

 than any of the others. These formulae 

 are sufficiently accurate for the most re- 

 fined work in the absolute measurement of 

 resistance. 



Remarkable Optical Properties of Carbo- 

 rundum: Lewis B. Jewell. 

 Some transparent blue plates of car- 

 borundum, found in 1904, gave by pre- 



liminary measures with a microscope a very 

 high value for the refractive index. 



Later, in material received from the Car- 

 borundum Company of Niagara Falls, were 

 found some transparent and colorless 

 plates, some of which were fairly thick. 

 The polarization of the plates was studied 

 with the polariscope and one specimen of 

 twinned crystals produced a well-defined, 

 transparent, colorless prism where the 

 plates were joined. 



Measurements with the spectrometer 

 were made and the refraction proved to be 

 greater than that of the diamond, and the 

 dispersion more than twice as great. 



Spectrograms were obtained of the light 

 transmitted through the plates and a thick- 

 ness of one thirtieth of a millimeter ab- 

 sorbed practically all light having a wave- 

 length less than 4,000 Angstrom units. 



Interference bands in the spectrum of 

 light transmitted through their parallel 

 plates gave greater values of the dispersion 

 than the measurements with the spectrom- 

 eter. Spectrograms have also been ob- 

 tained of the light reflected from the sur- 

 faces of the crystals and the reflective 

 power, which is extremely high, differs 

 very little in the visible and ultra-violet 

 portions of the spectrum until a wave- 

 length of about 2,400 is reached, beyond 

 which there seems to be an absorption 

 band. 



Very remarkable markings have been 

 found upon some plates, the most remark- 

 able of which take the form of spirals, 

 some of which are hexagonal and others 

 triangular, changing to hexagonal farther 

 out. Most of the spirals are, however, 

 circular ; some of them are so close together 

 and so regular as to form brilliant spectra, 

 even to the sixth or eighth order ; the lines 

 in some cases are as close together as thirty 

 or fifty thousand to the inch. Many of 

 these markings are extremely beautiful. 



Sometimes there are two outer coatings 



