Potentiometer Current.--A medium sized storage cell will be found desirable, producing a steady current. Errors in measurements are frequently made by using an unsteady current.
Setting for Standard Cell.--Set the standard cell to correspond with the certified pressure of the standard cell as given in its certificate. In using the potentiometer shown in [fig. 610], place the plug in hole 1, and see that it is always in this position when checking against the standard cell. Place the double throw switch at STD. CELL.
Adjust the regulating rheostat until the galvanometer shows no deflection. In making the first adjustment use the key marked R1; when a balance is almost attained, use key R2, and for the final adjustment use key marked R0. This cuts out the resistance in series with the galvanometer and gives the maximum sensibility.
Measurement of Unknown Pressure.--The potentiometer ([fig. 610]), as ordinarily used, gives direct readings for voltages up to and including 1.6 volts. For pressures higher than 1.6 volts, a volt box or multiplier should be used. After obtaining the standard cell balance, as previously described, place the double throw switch in the position marked E.M.F. The balance for the unknown E.M.F. is obtained by manipulating the tenths switch and rotating the contact on the extended potentiometer wire. The final position of the two contacts in conjunction with the position of the plug at the left of the instrument indicates the voltage under test.
As directed above, use key R1 for rough adjustment, R2 for intermediate adjustment, and key R0 for final adjustment.
Plug at 1 gives readings for voltage directly from settings of tenths switch and extended wire contact.
Plug at .1 shunts the potentiometer circuit so that the voltage measured is .1 of the reading taken directly from the scale. Hence, the readings taken from the setting of the tenths switch and the slide wire contact must be divided by 10.
To Balance Galvanometer for Unknown Voltage.--Place plug in hole 1 ([fig. 610]) for voltages up to 1.6, and in hole .1 for voltages up to .16. Rotate the tenths switch until a condition of balance is obtained exactly or approximately. To secure an exact balance, rotate the contact on the extended wire. The unknown voltage can now be read directly from the position of the tenths switch and the extended wire contact if plug be at 1, or by dividing by 10 if plug be at .1.
EXAMPLE.--A balance was obtained with the tenths switch at 1.3, the extended wire contact at 176 and the plug at 1. The voltage under test, therefore, is 1.3176. If the plug at .1 had been used, the same reading would have indicated .13176.
To ascertain if the current in the potentiometer circuit has altered during a measurement, it is only necessary to plug in at 1, place the double throw switch on STD. CELL and close the galvanometer key. No deflection indicates that the current has not changed. If the galvanometer deflect, the regulating rheostat must again be adjusted until the galvanometer shows no deflection.
To Measure Voltages from 1.6 to 16.--Pressures up to 16 volts may be measured by using a greater voltage across the BA posts ([fig. 615]). For this purpose a battery of about 20 volts should be used. Insert the large plug at .1 and throw the switch to STD. CELL, then balance the galvanometer by means of the regulating rheostat. When the rheostat has been set to secure a balance, insert the large plug at 1, set the switch on E.M.F. and read the voltage in the usual manner. Multiply the reading by 10.
Fig. 616.--Measurement of current with potentiometer. This is done by measuring the drop in volts across a known low resistance. In the figure S is the standard resistance, and on it are the pressure terminals pp, and the current terminals CC. The potentiometer is connected to the shunt through the posts marked P. The resistance between the points pp is adjusted to an even fraction of an ohm. These resistances are so chosen that in order to determine the current passing through the shunt, after having obtained a potentiometer balance it is only necessary to multiply the potentiometer reading by a simple factor. For instance, in using a .01 ohm standard. It is only necessary to multiply the potentiometer reading by 100, which gives the current reading in amperes; similarly, a .1 ohm requires multiplication by 10, and a .001 ohm by 1,000.
Care of Potentiometer.--The slide wire, although protected to a great extent by the hood, in time accumulates dust and dirt with a thin film of oxide. This will tend to increase the resistance in this part of the circuit owing to poor contact. This wire should, therefore, be cleaned occasionally.
To do this, unscrew the stop against which the hood strikes when turned to read zero; then remove the hood and rub the entire slide wire vigorously with a soft cloth dipped in vaseline. Do not use emery or sand paper as this will destroy the uniformity of the slide wire. Clean also the steel contact which rubs on the wire, as this becomes glazed after much use. When the potentiometer is not in use, the hood should be screwed all the way down, and the lid put in place to exclude dust.
If it be used in a chemical factory, laboratory, or any place where acid fumes are prevalent, this latter precaution is important, because the fumes may attack the slide wire.
It is also well to keep the contact surfaces of the switch studs clean and bright by wiping them occasionally with a soft cloth dipped in vaseline.
Res. of leads on each end is equal to 10 divisions of slide wire. The slide wire is divided into 1000 parts--20 for the leads, or 980 divisions. Calibrated scale on Galvanometer:
Fig. 617.--Diagram of Leeds and Northrup bridge for locating faults in power circuits, showing arrangement of the connections including the lead cables and galvanometer contacts. Make connections as shown. The clamps must be so fastened at A and C that the contact resistances will be very small. This contact resistance will figure as an error in the measurement. If, for instance, the contact resistance were equal to .001 of an ohm, and the wire were of such a size that .001 of an ohm were equal to the resistance of 20 feet of the cable, there would be an error of 20 feet in the location of the fault. For this reason all contact resistances throughout the loop from A to C must be extremely small. The battery is to be connected to the posts marked Ba., and the post marked Gr. is to be grounded. It will very frequently happen that the ground is to the cable sheath or some other conductor. In this case, the binding post Gr. should be grounded to this conductor. Sufficient battery should be used to give a readable deflection on the galvanometer for a small movement of the contact on the bridge wire. The fault is located by the usual Murray formula. If, for instance, the galvanometer show no deflection when the contact is at 300 on the scale, it would indicate that the fault is at a distance from A equal to .003 of the total length of the loop from A to C. A testing current of five amperes may be used with this bridge. In cases of necessity, this current may be increased to eight amperes, but when this current is used it should not be allowed to pass through the bridge for a longer time than is necessary. It frequently happens that small faults which have a very high resistance develop in high pressure cables. Such faults are likely to break down and result in damage and should be located. It is usually impossible to locate these faults until they have been partially carbonized. This must be done by applying a sufficiently high voltage between the cable and the sheath (or whatever it is grounded on) to break down the fault. In order to prevent the breaking down process from resulting in a serious burn out a high resistance must be placed in the circuit which will prevent an excessive current, or the circuit must be carefully fused. The former procedure is the better.
Location of Faults where the Loop is Composed of Cables of Different Cross Sections.--Faults in loops of this character may be located with the same degree of accuracy as those in loops of a uniform cross section, provided the length and cross section of each length of cable are known. An example will illustrate the method: