Proceedings, d&c, for 1886. 271 



Note on the Calibration of Am-meters and Volt-Meters. — In tests 

 such as those here mentioned it is very desirable to frequently cali- 

 brate our instruments. Fig. 4 illustrates a simple and effective 

 method of doing this. 



Consider the circuit represented by Fig. 4. Suppose the branch 

 A.C.B. is of resistance r, and contains an E.M.F., e. Suppose 

 the branch A.D.B. is of resistance R, and contains an E.M.F., E. 

 The branch A.B. of resistancy. 



^^. Er + eR ^ ^ ^, .. 



Then the current in A B is - p , ^ — [T^r"' ^"^ when this is 



r a + rp + rip 



R . ,, 



zero, then Er= — eR, orE=-e numerically. 



Let LL' (Fig. 5) be a wire conveying an unknown current through 

 the am-meter to be tested. We want to determine this current, and 

 thereby the value of the indications on the am-meter. 



Insert a known resistance, P, between the points W in LL, 

 and connect up two resistance boxes, a galvanometer, and a battery 

 of known E.M.F., as shown. 

 Adjust R and ?- till the galv. shows no current in A.B. 



Then we have 



E _ 5 -p 

 LL'- r bb' 



Where Ell' is the dif. of potential of the points I and I' = E 



say, and Ej,& is the known E.M.F. of the battery = e. 



_R 



^ r i_ R 6 6 



The current in W or LL' is therefore- = p— ^rp ^^ ^Tp '1^ 



Or, for example, if we make P = 0*1, e = 1, and r = 1000. 

 We have 



, . LL' 100 X 1 ^ R 



current in , — TTnTTr 'R 



I 1000 iOO 



Note. — Since R is assumed to be the resistance of the circuit 

 A.l.l'.B., it must be great compared to P, and the connections. 

 Similarly, r must be great compared to the battery and connections ; 



C ^W 



and in order that ,,, = ^- may be true, P must be small com- 

 pared with the total shunt resistance between I and l'. Otherwise, 

 the total resistance of each branch must be ascertained. The 

 material of which resistance P is made must be of such conductivity 

 as not to be sensibly altered by the passage of the heavy current, or 

 it must be measured while hot. It is obvious that the accuracy of 

 this method depends only on the delicacy of the galvanometer and 

 the accuracy with which the E.M.F. of the standard battery is 

 known. 



