86 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
galvanometer spot of 57 mm. (at 2 m.) per microvolt. To obtain a precision of 1 part 
in 100,000 on a single reading it is necessary, therefore, to note the change of 
deflection when the current is reversed with an error not greater than 11’4 mm. 
When the motion of the coil is made aperiodic by introducing additional external 
resistance, the sensitiveness is about one-quarter of the previous value, and the error 
of the difference reading must not be greater than about 3 mm. For measurements 
of a resistance of O'OOl ohm the difference of potential on the standard resistance was 
about O' 004 volt and the change of deflection had to be read with an error not greater 
than 0'6 mm. For a single measurement of a resistance about fifty different readings 
were taken and the mean of these is used to calculate the result (see p. 98). 
Fig. 18 is a simple diagram of the circuit and requires no explanation. Fig. 19 is 
a more detailed diagram and shows the connection of the circuit to earth. 
Section 21.—Standard Resistances. 
The resistances measured in absolute measure were of nominal values O'OOl ohm, 
0'002 ohm, and 0'01 ohm. The first and last of these were standard manganin 
resistances capable of carrying currents of 30 and 10 amperes respectively without 
increasing in temperature by an appreciable amount. 
The currents used in our measurements did not exceed 
4 and 2 amperes respectively, and the heating effect 
due to these currents could not be detected. 
To obtain an effect corresponding to that of a resist¬ 
ance of 0'002 ohm, three standard resistances were 
arranged in a triangular fashion (fig. 20) as first 
suggested by Lord Rayleigh.* The resistances 
are of the well-known type introduced by the 
Physikalisch-Technische Reichsanstalt; they are of 
manganin and the mean temperature coefficient in the 
neighbourhood of 20° C. is about +15 parts in 1,000,000 
per degree rise of temperature. The resistance (a) fig. 20 is a coil of 2 ohms 
resistance, (b) is a 1-ohm standard, and (c) consists of a coil of 1000 ohms shunted by 
others having values from 100,000 to 500,000 ohms. If the current in the main 
circuit is i the current through b is 
ai 
where L is the summed value of the 
ct + b + c + L > 
resistances of the current leads to the three coils. The difference of potential at the 
• r 7 • ccbi m, ab 
extremities of b is 
The quantity 
thus takes the place of R 
c< + & + c + L (x + 6 + c + L 
in the formula R = Mr (p. 35) and is called the effective resistance.! The two ohm 
* ‘Phil. Trans.,’ 1883. 
t A complete treatment on the combinations of a four-terminal resistance with other resistances is 
given by G. F. C. Searle, ‘ The Electrician,’ March 31 to April 21, 1911. 
