568 Mr. F. E. Smith on Bridge Methods for Resistance 

 and 



7 \g + 6 + Li + Jj 2 / _ ya + o + Lj+Lj 



From (18) and (19) we have 



(19) 



(20) 



b s 



Afterwards, by further adjustments of Q and a and of a or 

 /3, a balance is obtained which holds good whether L 3 is 

 connected to Q m /3 or not. In these circumstances 



a + /> + L 1 + L 2 _ a + fe + Li + La , 91 . 



Q + /3 ~ S 



and the relation (15) also holds. From (15) and (21) we 

 have 



R + 



a-fL 4 a + 6 + Li + Lg 



(22) 



(3 S 



and from (15), (20), and (22) it follows that 



P = QR/S (23) 



This preliminary setting of the bridge is, in practice, 

 comparatively easy, but if the arms of the bridge are 

 carefully chosen it is not necessary, even in precision work. 



For the purposes of thermometric measurements with a 

 thermometer of 1 ohm fundamental interval, it is convenient 

 to choose as nominal values : — R = 5 ohms, S = 500 ohms, 

 a = 9*85 ohms, /3=1000 ohms, and 6 = 500 ohms. Q and a 

 are equal variable resistances and are automatically changed 

 together. 



If the preliminary setting of the bridge is dispensed with, 

 the probable errors that are likely to be introduced, because 

 of it, are best seen from a consideration of the following 

 equation, which is another form of (16) 



QR /5L 3 /'R a+L 4 



/R _ a+L A 



a + /3 + L s + LAs /3 / 



. &l 3 17 Q a \ , / ^ L 3 lai 



"^a + ft + Lx + LaLvS &/ VS(" + /3 + L 3 + L,) b)\ 



.... (24) 



Let us suppose that measurements are to be made at 0° C. 

 Then the value of P is about 2'b ohms and Q and a will each 



