A Portable Millivoltmeter *

The instrument to be described was developed for the measurement of bioelectric potentials, especially those found on the skin. As such it was desired that it be portable, not dependent on the availability of electric current, and that, because of its biological applications, the input resistance be at least 10 megohms and that grid current through this resistance be less than 1 0-10 amperes within the 100 millivolt range of the instrument. The sensitivity required was 1 millivolt per scale division of a portable galvanometer (sensitivity about 0.040 microamperes per scale division). Other millivoltmeters1 utilize large power type tubes, with high grid-to-plate transconductance, operated at reduced voltages to achieve stability and low grid current. These circuits have several disadvantages where portability is of importance: first, because of their large current requirements, a heavy-duty B battery for the plate circuit and a storage battery for the filaments become necessary; second, despite the use of these large batteries, the high current drain still causes enough instability so that a balanced bridge circuit is needed to eliminate 'battery variations; third, in achieving the requisite stability, the bridge doubles the current requirements while halving the sensitivity of the instrument as compared to that of a single tube. While less sensitive tubes (lower transconductance) designed for dry cell operation may be chosen,2 they also must employ the balanced circuit with its attendant disadvantages. A new series of miniature tubest is well suited to portable battery powered apparatus. The filament draws less than 15 milliamperes from a single 1.5 volt cell. The plate current of the triode VW-32 is about 250 microamperes with a 10.5 volt B battery. Because of these low current requirements, it was found that flashlight cells for the plate supplv and a single No. 6 type dry cell for the filament