A new dendrometer has been developed for monitoring radial dimensional change of trees in tropical environments. The sensing unit of this instrument consists of a rectilinear potentiometer in a moistureand corrosion-resistant housing. The sensor is powered by a mercury battery through simple circuitry. Sensor output can be read with a voltmeter or recorded on a variety of instruments. SYSTEMATIC STUDIES of tree stem growth have been carried on for many years. Early investigators used handheld measuring tools such as tapes or calipers, and generally allowed growth periods of months or years between measurements (Marsham 1759, Mohl 1844, Christison 1879). These methods proved impractical, however, for assessment of the smaller growth fluctuations that occur during shorter time periods. This impracticality resulted mainly from the low precision of the instruments used and from the difficulty of reapplying them in the same position and with the same force for each succeeding measurement. Highly sensitive growth-measuring devices that could be permanently attached to the tree were first developed in the late eighteen-hundreds (B6hmerle 1883, Friedrich 1890, Reuss 1890). Instruments of this type are now generally called dendrometers or dendrographs. They were termed Zuwachsuhren, Zuwachsmesser, Stirkenzuwachsmesser, Zuwachsautographen, or Prazisionsbaummessinstrumente in the early literature. Dendrometers are designed to measure growth along a single radius, two opposing radii, or many radii of a plant part. They may therefore be called radial, diametral, or circumferential. These general types differ in such factors as area measured, sensitivity, bulk, and amount of abnormal growth induced. The amount of cambial area being monitored is generally least in radial dendrometers and greatest in the circumferential instruments. Radial dendrometers are therefore ideally saited for monitoring the activity of smaller portions of the cambial cylinder. Diametral and circumferential devices provide a larger sample that can be more indicative of total stem growth if the cross-sectional shape of the trunk at the monitored location is representative of the entire bole. Circumferential instruments are subject to friction between the bark and the band or wire that encircles the stem in this design. The friction limits their fidelity in following small growth fluctuations, especially those involving shrinkage (Bormann and Kozlowski 1962). Some designers have, however, alleviated this problem by placing rollers under the band. Diametral dendrometers are generally more bulky than other types since a rigid frame is required to span or encircle the stem. In most cases the frame necessitates a supporting mechanism which also adds to the bulk of the system. Many designs require that nails or screws be driven into the tree, either to support the instrument or to serve as a fixed reference against which growth is measured. This disturbance frequently evokes an abnormal growth acceleration in the vicinity of the injury (Fritts 1961, Abetz 1966, Mitscherlich et al. 1966). The amount of abnormal growth induced varies inversely with distance from the wound (Abetz 1966). Radial dendrometers are the most likely type to be affected by abnormal growth. Not only are nails or screws required to reference these devices, but they must usually be inserted near the measured area. Screws are also needed for some diametral and circumferential dendrometers. These instruments are, however, less affected by abnormal growth since the screws are used only for support and can therefore be placed farther from the area being measured. The various clamping mechanisms that have been used to support other circumferential and diametral dendrometers could also influence growth, but probably to a lesser degree. Comparisons of dendrometer types have been made by Fritts (1961), Bormann and Kozlowski (1962), as well as by Stamenkovic and Mis'devic (1971). Dendrometers are designed to indicate growth either mechanically or electrically. The mechanical type has been used most frequently and will be discussed first.