C. C. Hutchins — Boys Radiomicrometer. 249 



Art. XXVI. — The Boys Radiomicrometer; by C. C. 

 Hutchins. 



The following represents the result of many weeks of work 

 spent in the attempt to improve the radiomicrometer, invented 

 by Boys in 1888. If one may judge from notices in the scien- 

 tific journals, comparatively few have made use of the instru- 

 ment since its invention. Its simplicity, wonderful sensitive- 

 ness, freedom from outside disturbance and other excellent 

 qualities commend it highly, and were it not for the difficulty 

 of preparing the small circuit that is its active and vital part, 

 doubtless it would find a more extended use. Nor is it the 

 mere mechanical difficulties of construction which are here 

 implied; anyone with a fair degree of skill may overcome 

 them, and having done so, it is disheartening to find one's labor 

 in vain — as is very likely to be the case — so frequently do the 

 troubles caused by the magnetic peculiarities of the wire form- 

 ing the conducting loop make all skill of no avail. 



Lewis* made more than seventy-five of these circuits before 

 obtaining a usable one — a record of monumental patience, 

 but hardly calculated to inspire others to begin. The experi- 

 ence of others does not seem to have been very different. The 

 following instance will show how minute are the forces that 

 come into play. A circuit was made from ISTo. 36 copper wire 

 cleaned with fine sand paper. It was found so strongly dia- 

 magnetic as to be quite useless. A second circuit was made 

 from the same sample of wire, but cleaned with emery paper 

 that had previously been rubbed upon iron. This circuit 

 proved paramagnetic, and useless for that reason. It will, 

 then, be readily seen how, using ordinary materials, one might 

 spend weeks and never obtain a sufficiently neutral circuit. 



If the loop be paramagnetic it sets with its plane parallel to 

 the lines of force of the magnetic field and has a short period 

 of vibration, and the instrument is consequently insensitive. 

 If, on the other hand, it be diamagnetic, it sets with the plane 

 of the loop at right angles to the lines of force. If now it be 

 constrained by torsion of the suspension to take its proper 

 direction, unless it be very strongly diamagnetic, it does not 

 return at once to its position of equilibrium, but drifts back 

 and forth without ever coming to rest, and only after some 

 minutes or some hours returns to the position from which it 

 was disturbed. 



Many variations of the above program may be observed, 

 but such is their general behavior as I have observed it, and it 



* Standard wave-lengths in the infra red spectra of the elements, Astro- 

 physical Journal, 1895, ii, 1. 



