1 68 THE SCIENTIFIC PAPERS OF 



PART THIED. 

 ON A SIMPLE METHOD OF MEASURING ELECTRICAL RESISTANCES. 



RESISTANCE MEASURERS AND GALVANOMETERS. Although the 

 Wheatstone balance furnishes the electrician with the means of 

 measuring the resistance of electrical circuits with great accuracy, 

 provided only that reliable resistance scales and a delicate galvano- 

 meter are at hand, its application is, in many cases, rendered diffi- 

 cult on account of the delicacy of the apparatus and of extraneous 

 disturbing causes. 



In cases where a portable instrument is required which may 

 have to be entrusted to inexperienced hands, the want of a more 

 simple method of ascertaining electrical resistances makes itself 

 particularly felt. Having had occasion to require such an instru- 

 ment for measuring temperatures at inaccessible places, I pro- 

 jected,* some years since, a " resistance measurer," which has been 

 described by the Electrical Standard Committee of the British 

 Association, in their report, at Dundee, of 1867, and which is 

 based upon the power of balancing the potential values of two 

 equal coils upon a magnetic needle, by changing their relative dis- 

 tance from it, according to the intensity of the two branch currents 

 emanating from the same battery ; this distance being made the 

 measure of the unknown resistance inserted in one of the two 

 branches. 



Dr. Werner Siemens has produced a measuring instrument of 

 greater scope and convenience, in which an index handle (moving 

 a contact roller upon a wire in a circular groove) is earned round 

 upon a divided scale until a magnetic needle in the centre of the 

 apparatus assumes its zero position, when the unknown resistance 

 is indicated upon the scale. The same instrument is suitable for 

 measuring greater resistances by the sine method ; it is also a 

 tangent galvanometer, and has received the appropriate appella- 

 tion of a " universal galvanometer." 



These and other ready methods which have been projected for 

 measuring electrical resistances are useful auxiliaries to the Wheat- 

 stone bridge, from which they differ chiefly in obviating the neces- 

 sity of elaborate resistance scales, without, however, removing the 

 difficulty of dealing with a delicate galvanometer. 



