"Neutral hydrogen survey of andromeda galaxy": addendum.

mum) and the temperature coefficient of d-c output voltage is 1 mv per degree Celsius. A negative capacitance circuit is provided which improves the overall frequency response of the amplifier but unavoidably adds noise to the input signal if large stray capacitances are compensated. Signals of 40 pv through 7 Mohm (microelectrode in saline) can be faithfully reproduced. The amplifier has been tested in physiological experiments in which microelectrodes (0.5 to 3 IL) were used to record resting and action potentials (extracellular) from neurons in frog spinal cord, chicken cerebellum, isolated monkey brain (2), and nerve fibers in vitro. The IGFET amplifier gave consistently better overall results than a variety of commercial electrometer amplifiers (vacuum tube and transistor types) that were used to make the same measurements. Many other applications for this device and simple associated circuits have already been found. I believe the unusual features of this amplifier make it suitable for routine use in many types of research; it seems to answer the needs of large laboratory instruction classes where 10 to 20 amplifiers are needed and the cost of commercial devices is prohibitive. RONALD L. CECHNER Case Institute of Technology, Cleveland, Ohio 44106