Variable resistors

If the individual materials are separated by insulating couplings but connected to a protection system, the connections must be made through diodes to avoid bimetallic corrosion when the protection system is shut down (see Fig. 11-6). Furthermore, the different protection currents should be adjusted via variable resistors. 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Hodgkin and Huxley [81] formulated a membrane model that accounts for K" ", Na" ", and ion leakage channels in squid giant axons [Fig. 22(a)]. The membrane resting potential for each ion species is treated like a battery and the degree to which the channel is open is modeled by a variable resistor. 

The procedure is as follows. In switch position 1 and while repeatedly depressing tap key K, the variable resistor R, is adjusted once for each measurement to zero current through galvanometer G, so that the emf of the standard cell C8t (Weston 1.01832 V) becomes accurately compensated over the constant resistor Rj. Next, in switch position 2 the unknown emf of cell Cx is... 

Grain boundary defects are primarily responsible for the operation of zinc oxide (ZnO) varistors, a shortened form of variable resistor. The varistor behaves like an insulator or poor semiconductor at lower electrical field strengths, but at a critical breakdown voltage the resistance decreases enormously and the material behaves like an electrical conductor (Fig. 3.36). When a varistor is connected in parallel with electrical equipment, negligible power flows through it under normal low... 

We want to turn up the radio because it s noisy outside, and we want to hear what is broadcast. We therefore turn the volume knob toward loud . At its most basic, the volume control is a variable resistor, across which we pass a current from the battery, acting much like a kettle element. If we turn up the volume control then a larger current is allowed to flow, causing more energy to be produced by the resistor. As a listener, we hear a response because the sound from the speakers becomes louder. The speakers work harder. 

In practice, the dimmer switch incorporates a small variable resistor into the switch box, which allows differing amounts of voltage to be tapped off. ... 

Current is passed across the arc-gap in series with the help of a variable resistor C (10 - 40 2) and an inductance coil B. 

A simpler variation would be to use a normal Hall ignition system where you use the induction pulse to trigger a timing circuit to open the HV combined circuit for a specific period of time. The timing can be simple capacitor based or use a simple 555 1C timer with an adjustable variable resistor used to set the actual timing of the firing period. 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

Figure 15.3 —Ionisation devices, a) Continuous current arc (globular technique). The electrodes are inserted in a series circuit having a variable resistor, a cell and a continuous power source of a few tens of volts b) Glow discharge device using argon (hollow cathode lamp type). Samples can be introduced as powders or non-conductive pellets.

Two important parts of the electronics for a TCD are not showr in Figure 5.8. In practice the four cells of a detector rarely match. A control is usually provided so that the output voltage can be nulled before a chromatogram is run. This is usually done with variable resistors connected in the bridge. Often there are two controls on the front panel "Fine" and "Coarse" Balance. Another necessary control, at least when an integrator is not used, is an attenuator switch. This reduces the response on the recorder by various amounts so that the larger peaks can be kept... 

Zinc oxide is a semiconducting ceramic used to make varistors (variable resistors). Write a balanced equation for the hydrolysis of zinc ethoxide in the sol-gel method for making ZnO powders. 

Figure 8 shows a basic bridge circuit which consists of three known resistances, Rx, R2, and R3 (variable), an unknown variable resistor Rx (RTD), a source of voltage, and a sensitive ammeter. 

One known variable resistor (R3) that is used to match the unknown variable resistor... 

Other resistance-type transducers combine a bellows or a bourdon tube with a variable resistor, as shown in Figure 6. As pressure changes, the bellows will either expand or contract. This expansion and contraction causes the attached slider to move along the slidewire, increasing or decreasing the resistance, and thereby indicating an increase or decrease in pressure. 

Potentiometer valve position indicators (Figure 5) provide an accurate indication of position throughout the travel of a valve or control rod. The extension is physically attached to a variable resistor. As the extension moves up or down, the resistance of the attached circuit changes, changing the amount of current flow in the circuit. The amount of current is proportional to the valve position. 

The operation of proximity sensors can be based on a wide range of principles, including capacitance, induction, Hall and magnetic effects variable reluctance, linear variable differential transformer (LVDT), variable resistor mechanical and electromechanical limit switches optical, photoelectric, or fiber-optic sensors laser-based distance, dimension, or thickness sensors air gap sensors ultrasonic and displacement transducers. Their detection ranges vary from micrometers to meters, and their applications include the measurement of position, displacement, proximity, or operational limits in controlling moving components of valves and dampers. Either linear or angular position can be measured ... 

Figure 17 Configuration of op amp to function as a potentiostat. The desired Vin is set by the experimenter via the voltage divider on the left, which consists of a battery and a variable resistor. The path of the current is shown, indicating that no current passes through the RE.

Rote et al. (1993, 1994) used a carotid thrombosis model in dogs. A calibrated electromagnetic flow meter was placed on each common carotid artery proximal to both the point of insertion of an intravascular electrode and a mechanical constrictor. The external constrictor was adjusted with a screw until the pulsatile flow pattern decreased by 25 % without altering the mean blood flow. Electrolytic injury to the intimal surface was accomplished with the use of an intravascular electrode composed of a Teflon-insulated silver-coated copper wire connected to the positive pole of a 9-V nickel-cadmium battery in series with a 250000 ohm variable resistor. The cathode was connected to a subcutaneous site. Injury was initiated in the right carotid artery by application of a 150 xA continuous pulse anodal direct current to the intimal surface of the vessel for a maximum duration of 3 h or for 30 min beyond the time of complete vessel occlusion as determined by the blood flow recording. Upon completion of the study on the right carotid, the procedure for induction of vessel wall injury was repeated on the left carotid artery after administration of the test drug. 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

The choice between galvanostatic and potentiostatic measurements depends on circumstances. From the instrumentation point of view, galvanostats are much simpler than potentiostats. This is not only a matter of cost, but also a matter of performance. Thus, where it is desired to measure very low currents (e.g., on single microelectrodes), a battery with a variable resistor may be all that is needed to set up a low-noise galvanostat. At the other extreme, when large currents must be passed — for instance, in an industrial pilot plant for electrosynthesis - power supplies delivering controllable currents in the range of hundreds of amperes are readily available, whereas potentiostats of comparable output are either nonexistent or extremely expensive. 

The circuit supplying the exponential was powered by the square wave generator of the oscilloscope and the value of the decay constant was controlled by a capacitor and variable resistors in the circuit. Rate coefficients determined by this method agreed with those found by photographing the experimental trace and analyzing it graphically. 

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