104 Measurement of Alternating Currents. 
that this difficulty can be overcome, and I think that in any 
case too much importance should not be attached to it, 
Switchboard instruments can be calibrated in the vertical 
position, and are not likely to be used in any other. 
Laboratory standard instruments will naturally be cali- 
brated and used horizontally as usual, because the pivot 
friction is less in this position. | 
The heaters in these instruments are made of platinum 
alloys to stand the high temperature and not deteriorate. 
Each of the heaters is quite small, and would get easily into 
a space 6X6xX071 mm. That in No. 1 consists of a short 
piece of wire bent zigzag, and that in No. 2 of a piece of 
foil. The self-induction of these heaters is therefore very 
small, probably much less than that of any leads that can be 
used between them and the circuit in which the instruments 
are used. The instruments can be used to measure very high 
frequency currents. . 
The question of the permanency of the calibration is of 
course a vital one. Judging from our knowledge of the 
permanency of ordinary moving-coil intruments and the great 
stability of the platinum alloys used for the heaters, I see no 
reason to expect any serious alteration in these with time. 
The constancy of the thermo-junction and its thermo-force 
is open to question, which can only be satisfactorily answered 
by time. Up to the present the instruments are keeping all 
right. 
“The design of sufficiently non-inductive shunts to carry 
several thousand amperes for use with these and .other 
instruments is receiving attention from Mr. Mather and 
myself, and the matter is being experimentally investigated. 
I have no doubt that the shunts can be made, it is merely a 
question of the best design. of a swan 
The advantage of being able to use a single instrument 
like No. 2, which can be easily and accurately standardised 
with direct current in connexion with a set of standard 
shunts giving 0°15 volt drop for all currents from 2 amperes 
to several thousand amperes, will be appreciated by all who 
have any quantity of testing work to carry out. | 
The ease with which one can measure any direct current 
with a single good moving-coil ammeter and a set of shunts, 
and the facility with which one can check the accuracy of 
the instrument and shunts, have always greatly appealed to 
me, and the present instrument is, I venture to think, a 
step forwards in this direction in the case of alternating 
currents. 
