404 
DR. W. S. TUCKER AND MR, E. T. PARIS ON 
under tlie condition that the current remains constant. That is, iR is the change 
in resistance which would be measured if the bridge were re-balanced by inserting 
resistance in the microphone arm ; or if a bridge of the “ constant current ” type is 
used, c)R is simply proportional to the galvanometer deflection. 
The relation between ^R and R (the initial resistance of the grid), is a linear one, viz. : 
SR = 0‘2 (R—140), 
140 ohms being the resistance of the grid at air temperature. Therefore, by altering R— 
or, what is the same thing, by altering the heating current—the sensitivity of the grid 
can be varied in a perfectly definite manner. Con¬ 
versely, observations which have been made with 
the same microphone with different heating cur¬ 
rents can be very easily made to correspond by 
reducing them to some standard value of the 
current. If observations are taken under different 
conditions of air temperature, a correction on this 
account can easily be made if desired. 
A quantity which is more characteristic of 
the wire from which the grid is made is 
the change in resistance for a given sound in 
ohms per ohm. Probably the value of ~ for a 
given heating current and sound would provide the 
most convenient method of defining the sensitivity 
150 200 250 300 350 
/-? of a grid. The values of , obtained from 
_ it 
Fig. 8. 
the above table, are plotted against R in fig. 8, 
and show very clearly the way in which the sensitivity of the grid increases as its 
temperature rises. 
§ 6. The Resistance Changes in the Wire Grid. 
In this and the following two sections we shall examine more closely the means by 
which the platinum vdre grid is enabled to record electrically the aerial vibrations 
which are set up in the neck of the resonator. Suppose in the first instance that a 
microphone is held with its axis vertical (neck uppermost), and that the grid is connected 
in series with a battery and the primary of the first stage transformer of an amplifier. 
It is found by experiment that the temperature of the platinum wire, when carrying its 
normal safe working current of about 29 milliamperes, is in the neighbourhood of 
600° C., and we know that in these circumstances the energy supplied to the wire in 
