KELVIN BRIDGEFEATURES • • • Especially suited for measuring low resistances. Two or four terminal measurements can be performed. Lead Balancing not necessary when lead resistances lie within specified limits. This instrument employs the double Kelvin Bridge principle for accurate measurement of low resistances. Twin, 5-position ratio arm, combined with a decade & slide wire, provide a wide range of resistance measurement, i. e. 0.2 micro-ohms to 11 ohms with high accuracy. Two or four terminal measurements may be employed. Current up to 10 A can be used during measurement. All controls are simple to operate. Thermal e.m.f. may be taken into account using the three position (Normal/Off/Reverse) current switch. Both ratio arms are operated by one switch. The press keys give full and reduced sensitivity control for the galvanometer detector. Readings are obtained direct from decade and slide wire scale multiplied by ratio arm switch setting. All dials, keys and terminals are positioned on a instrument panel mounted in polished wooden case with hinged lid. of 0. PL39PS. whichever is greater. Conductor Clamp.2 micro-ohm 500 div.. of 200 micro-ohms 500 div. of 20 micro-ohms 500 div. TECHNICAL DATA: RANGES: Ratio Arm Decade Setting Resistance x ratio arm Setting 0. PL39. . No. of 2 micro-ohms 500 div. a mains operated 10A DC power supply.1 1 1. For convenience of operation.01 0.For measurement of resistance of thicker wires and rods up to 1 5 mm dia. Conductor Clamps are also available.05 % or 1 slide wire scale division. These Conductor Clamps have terminals for current as well as potential connections and are available for lengths of 100 cm and 50 cm.01 : suitable for wire/rod length 100 cm.1 milli ohm 10 Steps of 1 milli ohm 10 Steps of 10 milli-ohms 10 Steps of 100 milli-ohms 10 Steps of 1 ohm Slide wire resistance x ratio arm Setting 500 div. power: 230V AC.02 : suitable for wire/rod length 50 cm.0 100 10 Steps of 0. of 2 milli-ohms ACCURACY : ± 0. Cat. Conductor Clamp. PL 39. 50 Hz is also available. 00. The bridge is in two parts: a slide wire. and was made by Leeds and Northrup of Philadelphia. small differences between large resistances.. Carey Foster was Professor of Physics at University College. for example. Along with Alfred Porter of the same institution. he wrote "An Elementary Treatise on Electricity and Magnetism". The resistances hooked on the ends of the arms projecting up and down in the picture. In the 1903 L & N booklet describing the bridge there is an engraving of it with the name "Morris E. the name of the original company. Its cost was $150. . G. connected by thick copper cables to a holder for standard resistances. although it can be used to find. London. The Carey Foster Bridge in the picture is from Denison University. Phila. Leeds & Co.CAREY FOSTER AND KELVIN BRIDGES The Carey Foster bridge is often used for the measurement of very low resistances.". a standard text in the years around the beginning of the 20th century which I have found useful. founded on Joubert's "Traité Élémentaire D' Électricitié". W.00 in the 1927 Leeds and Northrup catalogue.2 percent. and is at Westminster College in western Pennsylvania .1 ohm with an accuracy to 0. It could be used to measure resistances in the range from 0. invented by William Thomson. Below is a similar Kelvin Bridge in the Greenslade Collection The Kelvin bridge below was made by J.001 ohm to 0. It is in the collection of the University of Vermont.The Kelvin Bridge is another form of low resistance bridge. The Students' Kelvin Bridge below was listed at $70. Queen of Philadelphia. Lord Kelvin. 0001 ohm.01 ohms. The device consists of a calibrated manganin rod with a maximum resistance of .The Kelvin bridge was designed to measure low resistances in the range of . Unknown samples 3/8" in diameter and 18" long can be measured. One potential terminal is fixed. . The 15 inch scale is divided into 100 equal divisions of .01 ohms.00001 to . the other is a sliding contact. All connections must be firm and electrically perfect so that contact resistances are held to a minimum. In using a Kelvin bridge. The use of point and knife-edge clamps is recommended. which is the circuit resistance existing between the standard and unknown resistances. the unknown resistance can be computed in the same manner as that for the Wheatstone bridge. Two additional resistances. is determined by the largest amount of heat that can be sustained by the bridge resistances without causing a change in their values. This value of current. shown in figure 3-1. A frequently used instrument that is capable of good precision is the Kelvin bridge. RS and R X. In performing the adjustment for balance. Note the similarity between this and the Wheatstone bridge. which affects the sensitivity of the bridge.001 ohm to 25 ohms.It is often necessary to make rapid measurements of low resistances. R1 and R2. such as samples of wire or low values of meter shunt resistors. Commercially manufactured Kelvin bridges have accuracies of approximately 2% for resistance ranges from 0. When this is done. . you must make the ratio of R1 to R2 equal to the ratio of RA to R B. A rheostat is usually placed in series with the battery so that bridge current can be conveniently limited to the maximum current allowable. respectively. because resistance R is effectively eliminated. you must follow precautions similar to those given for the Wheatstone bridge. are connected in series and shunted across resistance R. 1mW with degree high precision. This is because in Titian Kelvin was cleared touch effects (contact) and lead (lead) to obstruction when to measure little difficulty there are no known his value Rx . Telling how Kelvin's circuit different from Tetimbang Wheatstone from the aspect use? Kelvin's circuit is brow version Wheatstone was modified. 2.QUESTION 1. Scales Kelvin can be used to gauge obstruction in the range 1W . Kelvin bridges are well suited for what type of measurements? Small values of resistances. REFERENCE 1.edu/~dahammon/museum/kelvinbridge. http://www.com/content/neets/14193/css/14193_94.html 2. http://www.htm .uvm.tpub.