INTERNAL RESISTANCE OF A BATTERY 73 EXPERIMENT 35 INTERNAL RESISTANCE OF A BATTERY What is the effect on the current of decreasing the size of the plates of a cell and the distance between them? When the external resistance is small, what effect does it have on the current to arrange cells (a) in series and (b) in parallel? How can we measure the internal resistance of a cell? Daniell cell. Ammeter or low resistance galvanometer. Two dry cells. High resistance wire such as In this experiment we shall consider only cases where the external resistance is small. To measure the current we shall use an ammeter or galvanometer with low resistance. I. Effect of Internal Resistance on Current Furnished by a Cell. Connect a Daniell cell to an ammeter and observe the effect of bringing the zinc and copper plates (a) near together and (6) far apart. What effect on the internal resistance of a cell does it have to increase the distance between the plates? Gradually lift the plates out of the liquid and record the effect on the current. What effect on the internal resistance of a cell does it have to diminish the area plates immersed? II. When the External Resistance is Small, what Combination of Cells gives the Greatest Current? (a) Connect two similar cells in series with an ammeter and record the current. Compare this with the current furnished by one cell. How do the results of this experiment compare with results of testing the E. M. F. of two cells in series (Exp. 31)? (b) Join two cells in parallel and observe the current. Compare this with the current strength of one cell. How do these results compare with the E. M. F. test of two cells in parallel? How do you explain this difference? III. Measurement of Internal Resistance. Connect a Daniell cell with an ammeter and record the current. Introduce into the circuit some high resistance wire, such as No. 36 German silver wire, sufficient to reduce the current to just one half its former value. Measure the length of the German silver wire used and calculate from the specific resistance of the wire the resistance thus introduced. Assuming the E. M. F. of the cell to have remained constant, in order to reduce the current to one half, the resistance must have been doubled. This means that the internal resistance of the cell is equal to the resistance of the German silver wire which has been inserted. The internal resistance of cells arranged in series or in parallel can be computed just like the resistance of several wires in series or in parallel; that is, the series arrangement multiplies the internal resistance and the parallel arrangement divides the internal resistance of one cell by the number of cells. How should cells be connected to get a large current when the external resistance is small? When the external resistance is large? Problem. A telegraph sounder has a resistance of 70 ohms and requires 0.2 ampere to work it. How many gravity cells, each of 1.1 volts and 3.0 ohms, will be required? MEASUREMENT OF CURRENT 75 EXPERIMENT 36 MEASUREMENT OF CURRENT BY A COPPER How may an ammeter be checked by the weight of copper deposited in a certain time? The copper coulombmeter consists of a glass jar with two anode plates (A, A) and one cathode (C) or gain plate placed between them (Fig. 44). About 50 cm.2 of cathode surface is allowed for each ampere of current, and the liquid is a solution of copper sulphate (CuSO4), slightly acidulated with sulphuric acid (H2SO) and containing a little alcohol. The gain plate (cathode) is first made perfectly clean by rubbing with fine emery until bright, and then wiping with a clean dry cloth. After it is cleaned, the part which is to be immersed must not be touched by the fingers. Weigh this clean cathode as accurately as you can and set it aside. Connect the ammeter to be checked A a A Cathode FIG. 44 with an adjustable resistance in circuit with the coulombmeter and some supply of steady current such as a storage battery. Insert in the coulombmeter a trial cathode plate, not the clean one, but the same size as the one to be used. The current must be made to enter at the outside plates (anodes) and emerge at the middle or gain plate (cathode) s E (Fig. 45). Close the circuit and adjust the resistance to give the desired current (from 1 to 2 amperes). Open the circuit and replace the trial cathode by the clean weighed cathode and again close the circuit, noting exactly the time (hr. min. sec.). Record the ammeter reading every ten minutes and keep the current constant. After 30 or 40 minutes, break the circuit and at once remove the gain plate. Note the deposit of copper. Rinse off in clean water and then in alcohol and dry quickly. Reweigh and determine, the gain as precisely as possible. FIG. 45 Compute the gain in weight per hour. Assuming that 1.186 g. of copper is deposited by one ampere in one hour, compute the average current. Compare this value of the current with the average reading of the ammeter. Problem. How many ounces of copper would be deposited from a solution of copper sulphate in 10 hours by a current of 2.5 amperes? INDUCED CURRENTS 77 How EXPERIMENT 37 INDUCED CURRENTS may How may currents be induced by an electromagnet? currents be induced by means of a magnet? How may a conductor be moved in a magnetic field to generate a current? D'Arsonval galvanometer. 2 dry cells. 2 coils of about 800 turns No. 28 copper wire. Bar magnet. Soft iron core. Reversing switch. I. Induction by a Magnet. To see which way the needle of the d'Arsonval galvanometer turns when the current enters at the right-hand binding post, we may short-circuit the instrument with a stout copper wire and connect with a simple cell so that the current enters at the right terminal of the galvanometer. Place a piece of paper near the instrument and record the direction of the deflection with an arrow when the right terminal is made positive (+). Connect to the galvanometer (now without any shunt) a coil of many turns (say 800 turns of No. 28 copper wire). Galvano meter (a) Now move the coil downward quickly over the N-pole of the bar magnet (Fig. 46), and record the direction and amount of the deflection. From this deflection, determine the direction of the current induced in the coil. While this current was flowing in the coil, it made the coil a temporary magnet. What was the polarity of the side of the coil approaching the N-pole of the magnet? FIG. 46 |
