of Large and Small Alternating Currents. 93 
of all the other strands on itthe same. The suitable arrange- 
ment of the strands leads to the use of large cables and large- 
sized coils, generally consisting of a single turn. There is a 
further very serious obstacle to be overcome, namely, elimi- 
nating the effect of the magnetic field surrounding the cables 
which connect the instrument to the supply. There seems no 
reason why all these difficulties should not be overcome except 
the large size and cost of the instrument and considerable loss 
of power in it. 
Itis true that transformers can be used to bring the current 
within the easily measured range, but only at the cost of extra 
power losses in them and uncer rtainty i in the calibration unless 
calibrated for the particular frequency and wave-form on 
which they are to be used. In most cases, when it is required 
to measure very, small currents step-up transformers cannot 
be used owing to the power lost in them disturbing the circuit 
in which it is required to measure the current. Dynamo- 
meters cannot in general be shunted to use them for large 
currents unless the drop on the shunt is at least 3 volt, owing 
to the necessity of using a considerable dead resistance in 
series with the dynamometer itself to swamp its ees 
and temperature-coefficient errors. Such a drop as $ volt on 
the shunt corresponds to a considerable waste of power with 
large currents; thus with 2000 amperes to be measured the 
loss of power is 1000 watts, which requires a large shunt to 
dissipate it. 
The above remarks, which refer more especially to pure-air 
core-dynamometers, applv also to iron core or attracted iron 
type of instruments, with the further disadvantage of this 
latter class of instrument being subject to frequency and 
wave-form errors. 
(2) Electrostatic Instruments.—In this method the current 
is measured by measuring the P.D. between the terminals of 
a known non-inductive resistance through which the current 
flows. Using carefully designed non-inductive resistances, 
this is probably one of the most accurate methods of mea- 
suring alternating currents of any value, frequency or wave- 
form ; its chief limitation lies in the fact that up to the present 
no electrostatic voltmeter has been put on the market which 
can be relied on to measure accurately P.D.’s much below 
1 volt, and it is better to work with 4 or 5 volts. When 
measuring large currents a drop of 1 volt on the shunt is 
generally quite inadmissible ; and when measuring very small 
eurrents the P.D. in the whole circuit is often much less than 
1 volt. It must also not be forgotten that at high frequencies 
electrostatic voltmeters take an appreciable current which 
must be corrected for. 
