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



We have now to consider the best means of arrano-ino- the 

 bridge coils so that a shall equal &(R + Q)/(S — R) whatever 

 the value of Q. Since 6=?10), S=100, and R = lj we may 

 write 



100 (Q + l) 



9y 



Q + i, 



0-99 



If, therefore, the a coils be made equal to ^^ times the Q 



coils, and in addition a 1/0*99 ohm coil be placed so as to be 

 always in series with them, it follows that if the a and Q 

 coils are arranged in dials so that one cannot be varied with- 

 out the other, the relation a=?6(R4-Q)/(g-^R) must always 

 hold, 



Ffr, 12, 



Fig, 12 shows the general arrangement of the bridge. 

 The resistances Q and a are arranged in six dials with good 



brush contacts, an a coil being equal to q^k times the corre- 

 sponding Q coil. S consists of two coils in series, one of 

 99 ohms and one of 1 ohm. The object of thus splitting S 

 into two parts is to be able to rapidly check the bridge to 

 see if the equation 



a=£(R + Q)/(S^R) 



is sufficiently true for all possible values pf Q. To jnake the 

 check, the position of the current lead is changed from A to 

 C and short-circuiting straps of copper connect terminal L 2 

 to Gr 2 and L L to L 3 and G x . Fig, 14 now shows the scheme 



