The Rheostat

Manual rheostats can be used in series with the motor armature for the current-limiting func tion. If the rheostat has ample thermal capacity, it can also Be usedto vaiy speed. If this system is used, interlocks should be included to prevent closing of the contactor unless maximum resistance is in the circuit. 

Set the rheostat to its maximum resistance. Switch on the current from the mains. Bring the second silver electrode, holding... 

A resistance of 10-12 ohms is required for the apparatus described. It may be necessary to adjust the rheostat several times during the course of the reaction. 

Arrangement of the apparatus. This is shown diagrammatically in Fig. 53. The alternating current from the induction coil I flows to the contact C and to the wire joining the rheostat R with one of the plates of the cell X, there dividing into two currents, one flowing through the known resistance R and the part a of the wire and the other, by means of two copper wires... 

Procedure. Some of the wine is vigorously shaken to expel most of the carbon dioxide present and, after standing, exactly 20 c.c. are made up with water to 100 c.c. in a measuring flask. Of this diluted wine, 25 c.c. are introduced into the cell, care being taken that no gas-bubbles remain adherent to the electrodes. The cell is then placed in the thermostat and introduced into the circuit. When the temperature of the electrolyte has become constant, the induction coil is started and the resistance of the rheostat regulated (with the cell described, about 150 ohms will be... 

After the apparatus is assembled, 600 cc. of 20 per cent sulfuric acid (Note 3) is placed in the battery jar, and the lead anode in the porous cup is also covered with acid of the same strength. The cell is surrounded by an ice-salt bath, the stirrer is started, and, while the solution is cooling, 50 g. of nitrourea (Note 4) is added to the catholyte. A thermometer is placed in the catholyte, and when the temperature drops to +5° the current is turned on as follows with the rheostat set for maximum resistance, the current is switched on, then the rheostat is gradu-... 

Step 2. Set the rheostat to boil the water gently. Distill the water sample and collect 11 mL or more of the distilled water in the condensate collection vessel. 

Explanation of the term "transistor" as "transfer resistor" the constant job of Mr. Transistor Man for a common-emitter npn transistor is to turn the rheostat on the right, as needed, so that the equation Ie — PIb (with a preset value of the current gain / , say / =100) is always obeyed he makes sure that the ammeter on the right registers a current lE that is always a fixed amount ft times larger than current lB measured by the ammeter on the left. Adapted from [16]. 

The most common type of stirrer is one which uses an electric motor as the driving mechanism. These motors have considerable torque, their speed may be controlled easily, and they are very dependable in their operation. Many designs are available, but the one which has proved to be most generally useful is that of Hershberg. A %0-hp motor is connected to the driving shaft with a V belt via two pulleys which serve to reduce the speed and increase the torque. The speed of the motor is easily regulated with the rheostat which is attached to the stirrer housing (Fig. 5-9). 

House current is much too strong for such a short length of nichrome wire or flashlight battery carbon rods. Without the rheostat, the insulated wire would melt or a fuse would be blown. 

Constant speed direct-current motors are usually started by decreasing the amount of an external resistance in the armature circuit. The starting current may exceed full-load current by about 50 per cent. From 15 to 30 sec. should be allowed to bring a motor up to speed. This may be insured by automatic operation. The rheostat may incorporate low-voltage and overload releases, and should contain the motor-field circuit closing mechanism. Ordinary rheostats (starters) cost from 3 to 6 per cent as much as motors, the smaller sizes costing relatively more. 

In series with this is an adjustable rheostat CB. The maximum value of this resistance is chosen equal to the maximum value of the resistance of the line and couple likely to occur in practice. A convenient value is 15 ohms. With the slide of the rheostat set for the maximum resistance, ri = the instrument is calibrated in terms of the potential drop across AH. Hence when the instrument is connected through the line of resistance L to the couple of resistance T... 

Close tlic key and adjust the rheostat V] until the instrument reads approxi-mat.cdy the same as in 1. 

In 1877, he constructed a machine (called the Rheostatic Machine) in which he used multi-cell battery and a bank of capacitors and, with the help of a series of commutators and contacts, alternately charged the capacitors connected in parallel and then discharged them connected in series (Fig. 1.4). The large-scale model of this machine comprised 80 capacitors and a series of... 

It is important that the rheostat should not turn in one step from the SO-megohm value to the O.S-Mgohm value. Limit switches, backed by mechanical stops, prevent this. Because of the stops, a friction clutch, is inserted in the rheostat shaft. 

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