The weight voltameter The gas voltameter

GALVANOMETER

ANODE

GALVANOMETER

ANODE

RESISTOR

FIG. 5—Diagram of connection to obtain current strength by use of a silver-voltameter. Example: What is the current strength in amperes when after 20 minutes the silver deposit on the cathode is found to be 1.34 gram.

RESISTOR

FIG. 5—Diagram of connection to obtain current strength by use of a silver-voltameter. Example: What is the current strength in amperes when after 20 minutes the silver deposit on the cathode is found to be 1.34 gram.

Ct 1.118X20X60

=1 ampere

A typical silver voltameter is depicted in fig. 5 and functions principally as follows: The cathode on which the silver is de-

posited consists of a platinum bowl not less than 10 centimeters in diameter and from 4 to 5 centimeters in depth.

The anode consists of a disc or plate of pure silver, some 30 square centimeters in area and 2 to 3 centimeters in thickness, which is horizontally supported in the electrolyte near the top %by a silver rod rivited through its center.

To prevent the disintegrated silver which is formed on the anode falling upon the cathode, the anode should be wrapped around with pure filter paper, secured at the back by suitable folding.

The liquid consists of a neutral solution of pure silver nitrate (Ag NO?,) containing approximately 15 parts by weight of the nitrate to 85 parts of water.

i Direct Current Meters.—Most electrical measuring devices are fundamentally Current measuring devices, being either voltmeters, milliammeters or microammeters.

Construction.—Such a meter consists of a horseshoe magnet between the two poles of which is suspended an armature to which is attached a pointer and a spring arrangement to hold the pointer to its zero position when no current is being passed through the meter coil.

How the Current is Measured.—When a current be passed through the armature coil, it becomes an electromagnet, with two poles of opposite polarity, and the reaction between the energized coil and the permanent magnet causes the coil to rotate on its axis so as to facilitate the attraction of the unlike poles and the repulsion of the like poles of the two magnets. '' The amount of movement is determined by the balance ' attained between the resiliency of the spring mechanism and the strength of the magnetic field set up around the coil, and since the strength of the magnetic field set up around the coil is determined by the amount of current flowing through it, the movement may be calibrated in unit of currents, or in any other unit such as volts, ohms or microfarads, all of which possess a definite relationship to the unit of current.

FIG. 6—The essential parts of the instrument are: A, spiral spring; C, coil; K, soft iron core; M, permanent magnet and P, pointer. Current passing through the coil causes the moving system to turn against the restraining force due to the influence of the permanent magnet.

Connection of Meters.—A meter calibrated for current measurement in terms of amperes or fraction thereof, usually has a comparatively low resistance and is connected in series with the circuit in which the current is to be measured, whereas a potential pressure measuring meter or voltmeter is of comparatively high resistance and is connected across the circuit, across which a potential pressure is to be measured.

Direct Current Ammeters.—The ammeter as already described, is an instrument of low resistance, and is always connected in series with the current it is desired to measure. It is for this reason that the series resistance usually found in voltmeters are omitted.

AMMETER VOLTMETER

AMMETER VOLTMETER

FIG. 7—Indicating method of connecting for determination of current flow and pressure in a circuit.

Ammeters are employed for current measurement in all * branches of electrical work and may be designed for measurement of from a few milliamperes up to thousands of amperes. A milli-ammeter therefore is an ammeter which measures divisions of currents in ± qqq of an ampere, whereas an ammeter employed in measurements of larger amounts of current measures amperes in units of 1 to 100 or more.

Using a Milliammeter as a Voltmeter.—As previously pointed out the only difference between a voltmeter and a milliammeter is that a voltmeter has a high resistance connected in series with the moving coil.

Hence by connecting accurately fixed resistances in series with the milliammeter, it is possible to make a very useful voltmeter that may be employed to read filament voltages, plate voltages, C voltages, the output voltage of 5-power units, etc.

Of course it is self evident that the accuracy of such a converted meter depends upon the accuracy of the milliammeter, and the fixed resistance used.

The table shown gives the value of resistances required with different milliammeters to read voltages from 1 to 1,000 volts.

VOLTAGE MULTIPLIER FOR MILLIAMMETERS

Milli-Amperes

1,000 Ohms

10,000 Ohms

100,000 Ohms

1,000,000 Ohms

1.

1. volt

10 volts

100 volts

1000 volts

1.5

1.5 "

15 "

150 "

2.

2.

20 "

200 "

3.

3.

30 "

300 "

5.

5.

50 '

8.

8.

10.

10 "

«

Example.—If a 5 milliampere meter is to be employed to read voltages up to 50 volts, what resistance should be used?

Solution.—From table, the resistance required is 10,000

7 0.005

from which R = 10,000 as already obtained from table.

Likewise, if a 1-milliampere meter is to be used to read voltages up to 1,000 volts, then a 1-megohm resistance is placed in series with it.

If the values of resistance required to read voltages is not found in the table, the resistance may be obtained by calculation in the same manner as that already shown.

Resistors with a wattage rating of one watt will be satisfactory for all those values given in the table, however, it is advisable to use resistors with a rating of approximately 5 watts so that there will be little possibility of the value of the resistance changing due to the heating effect (PR).

Also, resistors with a 5 watts rating operating considerably below their rated dissipation, will be likely to hold their calibration a longer time then resistors of lower wattage.

The Direct Current Voltmeter.—Since the current through a meter is proportional to the voltage impressed at its terminal, any ammeter as previously described may be used as a voltmeter.

In this case however, a resistor of high value must be connected in series with the movable coil, because if an ammeter were connected directly across the line, it would immediately burn out due to the low resistance in its coil.

The high fixed resistance connected in series with the moving coil is considered as part of the meter.

Assume that the moving coil milliammeter, as used in the previous example, is to be utilized for a voltage measurement of 110 volts at full scale deflection, the allowable current drain to be 1 milliampere, what will be the value of the series resistance?

It is evident that the resistance unit must be of such a value that when the voltage across the terminal is 110 volts, exactly 1 milliampere will flow through the resistance and meter coil at full scale deflection of the pointer.

By Ohm's law is obtained:

= 110,000 ohms

VOLTMETER (2-Scales)

VOLTMETER (2-Scales)

As the moving coil resistance is very small compared with the series resistance, it may readily be omitted for most practical problems.

The series resistance or multiplier resistance may be tapped at various places to obtain more than one voltage range, and is usually placed inside the voltmeter case and connected in series with the Coil. See fig. 8.

If the voltmeter' with the 110,000 ohms series resistance be tapped at its center, the voltage range for the same current drain would be £=0.001x55,000 = 55 volts.

In order to obtain proper needle deflection the binding posts of the meters are marked + (plus) and - (minus). The post marked + should always be connected to the positive of the line and either of the other binding posts to the negative side.

+ + ?-!-++ - + + + + ■»-+-10». 50». 100». 250». 500». 1000». 10». 50». 100». 250». 500». 1000».

PIGS. 9 and 10—Two methods of connecting multipliers to a voltmeter. In fig. 9, one resistor is tapped at the various points to obtain the proper multiplier values for each scale as shown. This arrangement obviously is economical in that only one resistor need to be used. However, the disadvantage being that if an opening occur for example to the left of the 10 volt tap, the voltmeter will be rendered useless until the fault is being repaired, whereas if an opening occur within a resistance when connected as shown in fig. 10, only that particular scale will be effected.

+ + ?-!-++ - + + + + ■»-+-10». 50». 100». 250». 500». 1000». 10». 50». 100». 250». 500». 1000».

PIGS. 9 and 10—Two methods of connecting multipliers to a voltmeter. In fig. 9, one resistor is tapped at the various points to obtain the proper multiplier values for each scale as shown. This arrangement obviously is economical in that only one resistor need to be used. However, the disadvantage being that if an opening occur for example to the left of the 10 volt tap, the voltmeter will be rendered useless until the fault is being repaired, whereas if an opening occur within a resistance when connected as shown in fig. 10, only that particular scale will be effected.

How to Arrange Resistors for a Multi-Range Voltmeter.—

Resistors for multi-range voltmeters may be arranged in various ways as shown in figs. 9 and 10. Each resistor will give a certain definite voltage drop, and should be of the so-called precision type, unaffected by nominal temperature changes.

Voltmeters suitable for radio work usually have a resistance of 1,000 ohms per volt.

Inspecting the resistance arrangement in fig. 10 it is found that when using the 0-100 volt the circuit resistance is 100,000 ohms and when using the 0-250 volt scale, 250,000 ohms, etc.

VOLTMETER

VOLTMETER

-250,000 0HMS-

250 VOLTS -

-250,000 0HMS-

250 VOLTS -

X OHMS

SERIES RESISTANCES

-750 VOLTS-

PIG. 11—Illustrating how to increase voltmeter range by adding series resistance.

To compute the resistance to be inserted to obtain a certain voltage range, no difficult mathematical formulas need to be employed.

According to Ohm's law E = lxR; hence to obtain a voltage drop of 100 volts for example, with a current drain of 1 milli-

ampere, a resistance of "q^ooT or ohms should be inserted.

Example—Assume that the voltmeter shown in fig. 11 which has a range of 250 volts and a resistance of 250,000 ohms must be changed so as to enable it to be used on 1,000 volts, what value must a resistance connected in series with the existing multiplier have?

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