360 



Mr. E. Wilson. The Kelvin Quadrant 



In the Siemens Laboratory ttiere are two altera ate current machines* 

 coupled together in such manner that any desired phase difference 

 between their armatures can be obtained. In fig. 2, Mi and M a 

 represent the armatures of these machines. On the shaft of one of 

 these alternators is fixed a revolving contact maker, M, which makes 

 contact between two brushes once in a period, that is six times in a 

 revolution of the alternator, since there are twelve poles. It consists 

 of a gunmetal disk keyed to the shaft of the alternator, and carrying 

 two rings, one of ebonite and the other of gunmetal insulated from 

 the disk by means of the ebonite ring. Into the ebonite ring are in- 

 serted six contact-making strips of gunmetal one-sixteenth of an inch 

 thick, equally spaced out on the circumference and soldered into the 

 gunmetal ring. An insulated copper brush bears on the gunmetal 

 ring, and an insulated steel brush bears on the surface of the ebonite 

 ring, touching each of the contact-making strips as the contact 

 maker revolves. The epoch at which such contact is made by the 

 small steel brush can be varied and observed by means of a pointer 

 moving over a fixed circle divided into 360 equal parts. The 

 diameter of this revolving contact maker is 13 inches. 



Qi is the No. 184 electrometer used as a Wattmeter. 



Q 2 is the No. 71 electrometer used in connection with the revolving 

 contact maker M for the purpose of determining the instantaneous 

 values of the current and potential difference. 



D is a Kelvin balance or Siemens electro-dynamometer for the 

 measurement of current ; E is the thick wire circuit or circuits of 

 the "Watt-hour meters being tested ; P is a Kelvin multicellular 

 voltmeter ; r x , r 2 are non-inductive resistances of comparatively large 

 value for the purpose of reducing the potential difference applied to 

 the electrometer Q 2 , when measuring potential difference C ; the 

 pressure circuits, P, of the Watt-hour meters are placed across 

 ri + r 2 ', r 3 , r 4 are made up of a manganin strip 50'8 mm. wide and 

 0*4 mm. thick ; r 3 = r 4 = 0*2275 ohm at about 10° C. ; r 5 , r 6 are 

 non-inductive resistances of considerable magnitude for reducing 

 the potential difference applied to Q 2 when necessary. The junction 

 between r 3 and n is connected to the case of Q t ; the quadrants of 

 this instrument are connected respectively to the extreme ends of 



n ; whilst the needle of the electrometer is connected to the other 

 pole of M x . In connection with Q 2 , S x is a two-way switch for 

 observing potentials across r 2 or r 5 ; S 2 is the ordinary switch sup- 

 plied with the electrometer which short-circuits the quadrants when 

 moved to its central position, and in its two other positions reverses 

 the charge on the quadrants ; Gr is a condenser, which can be varied 



# A full description of these machines is given in the ' Phil. Trans.,' A, toI. 187 

 (1896), p. 231. 



