Journal of Neuroscience Methods, 3 (1980) 37--48 © Elsevier/North-Holland Biomedical Press 37 THE ELECTRICAL CHARACTERISTICS OF CARBON FIBRE MICROELECTRODES K. FOX, M. ARMSTRONG-JAMES and J. MILLAR Department of Physiology, The London Hospital Medical College, Turner Street, London E1 2AD (U.K.) (Received March 17th, 1980} (Accepted May 19th, 1980) Key words: recording microelectrodes -- carbon fibre microelectrodes -- iontophoresis The impedance of carbon fibre-containing microelectrodes was measured at a range of frequencies. From this and other data an equivalent circuit model for the microelectrodes was constructed. Various parameters of the model that contribute to the overall noise performance of the microelectrodes when recording extracellular nerve action potentials were discussed. INTRODUCTION A recent paper (Armstrong-James and Millar, 1979) described the con- struction of microelectrodes containing a single carbon fibre (diameter 7--8 pm) in the recording barrel. These microelectrodes appear to have similar recording qualities in the central nervous system to tungsten microelectrodes (Gesteland et al., 1959; Frank and Becker, 1964; Merrill and Ainsworth, 1972) but are easier to construct, and can be used in multibarrel microelec- trodes in iontophoretic studies (Curtis, 1964). In the present paper, we describe some of the electrical characteristics of these microelectrodes. METHODS Resistance of carbon fibres Single carbon fibres were removed from large bundles using fine jeweller's forceps and laid across a piece of clean wax-coated laboratory paper with their ends dipping into two small mercury puddles. (The fibres may be ob- tained from Courtaulds, Carbon Fibres Division, P.O. Box 16, Coventry, CV6 5AE, U.K.). A DC signal of a few millivolts was applied across silver wires dipping into the mercury and the resulting current read off with an accurate ammeter (Solartron 7040). The distance between the mercury puddles was measured and hence resistance per unit length of the fibres ob- tained.