
the Measurement of very small Alternating Currents. 601 
send a small current through the ammeter wire, and this is 
adjusted until the terminal ee difference of the ends of 
the ammeter wire is either 2, 4, or 6 volts as required. Let 
this last potential-difference be called V, and the P.D. down 
the resistance R, be called V5. Also let the resistance of the 
ammeter wire under the working circumstances be R. Then 
V/R is the current through the ammeter wire and V/R’ 
the current through the divided resistance, and the sum 
V/R+V/R’ is equal to V./R, which is measured. Hence 
R can be determined corresponding to various values of 
V/R. 
In the case of the actual calibration of the hot-wire 
ammeter already made, the resistance R' was 497 ohms, and 
hence the resistance R was obtained from the equation 
V a Vou Ve 
RB (49d dts 
The following table gives the observed values of V, Vo, Ra, 
and R,, and the calculated values of the wire-resistance R and 
eurrent A for the two platinoid wires (1) and (2) in the 
instrument already made. 



TABLE I, 
| | Wire (1). Wire (2). 
‘ame 
‘| V (in volts) ......... | 2 4 6 | 2 4 6 
Vv 
. ep | 0639 | 1:277 | 0°9575 | 0-634 | 1:2675 | 0°9500 
R, (in ohms)......... | 6372 | 3191 | 2070 || 6415! 3212] 2088 
= he 461 ~ 40 20 || 40| 40 20 
rele (in amperes). 0°0159 | 0-0319 | 0-0478 | 00158 0-0317 | 00475 
. i 0119 | 0:0288 | 0°0357 |0°0118 0:0236 | 0:0354 
R (cale.) in ohms...! 168°3 | 167°3 | 167°8 } 169°8 | 169°1 | 169°3 | 




Between the above limits of current 0°01 and 0:04 ampere 
the resistance of wire (1) may be taken as 168 ohms and 
that of wire (2) as 169°4 ohms. 
As an instance of the use of this ammeter and also of the 
precautions necessary in using it, the following measurements 
of the capacity of leyden-jars taken with it may be given. 
Two leyden-jars were employed which will be called 
respectively no. 1 and no. 2. A small alternating current 
transformer having a transformation ratio of 1 to. 118 was 
employed to step up the voltage of an alternator having an 
electromotive force curve of nearly sinple sine form. 
The jar to be measured was connected across the high- 
tension terminals of the transformer, and the hot-wire 
