Apeil 5, 1907] 



SCIENCE 



529 



of the rays incident upon it was examined 

 for sheets of silver, platinum, copper, tin, 

 and aluminium, and in every case it was 

 found that the effectiveness of a sheet as 

 an absorbing medium is independent of the 

 intensity of the incident rays. 



It was found that the effect of the sur- 

 faces of metallic sheets upon transmission 

 is small in the ease of copper and of alumi- 

 nium. To show this, a laminated plate of 

 the metal in question was prepared, equal 

 in total thickness to another solid plate of 

 the metal. These two plates were inter- 

 posed in turn in the path of the rays, and 

 produced equal reductions of the deflection 

 of the radiomicrometer. 



The transmission of a beam of Rontgen 

 rays through a metallic sheet has generally 

 been supposed to render the beam more 

 penetrating toward a second sheet of the 

 same or of any other metal than the origi- 

 nal beam was. 



Evidence of transformation of one sort 

 of ray into another, in transmission 

 through a metallic sheet, was sought, with 

 a negative result. 



The Mutual Inductance of a Circle and a 

 Coaxial Helix. The Lorenz Experiment 

 and the Ayrton-Jones Absolute Electro- 

 dynamometer: E. B. Rosa. 

 The constant of the Lorenz apparatus for 

 the absolute measurement of resistance and 

 of the Ayrton-Jones electrodynamometer 

 for the absolute measurement of current 

 require the calculation of the mutual in- 

 ductance of a circle and a coaxial helix, the 

 circle in the first case being the edge of the 

 rotating disc and in the second case one 

 end of the suspended coil, the helix being 

 the fixed coil carrying the primary current. 

 Jones obtained an expression for the 

 mutual inductance in question which is a 

 very tedious and difSeult one to use in 

 numerical calculations. Lorenz 's expres- 

 sion is an algebraic series less difficult but 



also less accurate than Jones's. Using an 

 expansion in zonal harmonies, I have ob- 

 tained an impression in the foi'm of an 

 algebraic series, similar to Lorenz 's but 

 more accurate, which is far more conve- 

 nient to use than Jones 's and agreeing with 

 the latter to less than one part in a million 

 (in a particular numerical test) thus being 

 amply accurate for the most refined experi- 

 mental work. This expression is obtained 

 on the assumption that the current is dis- 

 tributed over the solenoid in a uniform 

 current sheet, whereas Jones's expression 

 assumed the current flowing in a helix. 

 Their agi*eement is a confirmation of the 

 theorem that a spiral distribution of cur- 

 rent is equivalent to a current sheet. 



Mutual Inductances for Laboratory Use: 



E. B. Rosa. 



The most accurate as well as the most 

 convenient method of calibrating a ballistic 

 galvanometer is by means of a mutual in- 

 ductance, through the primary of which a 

 measured current flows. Some labora- 

 tories use a standard solenoid with a 

 secondary wound within or without, cal- 

 culating the mutual inductance of the pri- 

 mary and secondary coils from their di- 

 mensions. Such a solenoid made large 

 enough and carefully enough to give the 

 mutual inductance with fair precision is 

 both bulky and expensive, and not very 

 portable. Being of considerable length, its 

 magnetic field often extends to a consider- 

 able distance and may disturb other work. 

 On the other hand, suitable primary and 

 secondary coils may be wound on a thor- 

 oughly seasoned wood spool (or a marble 

 spool) only 10 to 12 cm. in diameter and 

 afford, when calibrated, an accurate and 

 convenient mutual inductance for the cali- 

 bration of galvanometers that is cheap, 

 portable and reliable. Such inductances 

 are so inexpensive and oceupyso little space 

 that one can be left with its secondary in 



