Clip leads

Battery, nine volts t Battery, AA, 1.5 volts Clip leads, 14 inch, one set. 

Before beginning the experiments, a few procedural things have to be covered. The source of electricity will be a 9 volt battery, and the connections will be made through clip leads. (The latter word is pronounced "leed," not "led" like lead metal would be.) In supply catalogs, the clip leads are sometimes described by other phrases such as "test leads," "jumper cables," or "patch cords." Because of their appearance, the adjustable connectors at the ends are called "alligator clips."... 

Connect the other end of that same clip lead to either one of the two secondary wires on the transformer, which both have thin yellow plastic insulation on them. Do not use any of the black wires of this transformer, either the thinly insulated "center tap" of the secondary coil or the two thickly insulated black wires of the "primary" coil. (This experiment can be done either with or without a long "power cord" and plug attached to the primary. )... 

At the upper end of the secondary coil, the symbol that is labeled "switch" in the diagram represents a contact that is made and then broken, repeatedly. It could be a real switch, such as you would use to turn on the lights in a classroom, but to save money we will just use one end of a clip lead that is touched for a short time to the upper transformer secondary wire. 

Now attach both wires of the neon tester to both terminals of the 9 volt battery, either with or without clip leads. (Of course, this is not a dangerous part of the experiment, since 9 volts is very unlikely to hurt anyone with reasonably dry skin, but it still makes good sense to avoid any two-handed contact of metal parts having voltage on them.) The neon bulb does not light up, even faintly. 

Fig. 1.3, using clip leads. The black dot symbol inside the circular bulb symbol means that there is some neon gas inside the bulb, and not an extreme sort of vacuum. A different black dot symbol occurs at the places where three wires come together, and this means that the wires are mutually connected together electrically. (This is in contrast to a possible situation, which you will see later, in more complex diagrams, where two wires cross but are not making electrical contact, because of insulation layers — no black dots are used then.)... 

It would be a good idea to use a red clip lead from the positive terminal of the battery to the neon tester lug. Then one end of a yellow clip lead is also attached to that same neon tester lug, but the other end of the yellow lead is not hooked up to anything yet — it is going to be the equivalent of the switch. A white clip lead can go from the other lug of the neon tester to the upper yellow wire of the secondary coil (that is, upper as drawn in the diagram, but not really up or down). The bare metal tip of that same upper secondary wire can then be momentarily touched by the unattached end of the yellow clip lead. 

In electrical terminology, when the metal wire is touched to the metal end of the clip lead, contact is "made," or we could say that the makeshift switch is "closed." Later, when the metal parts are moved away from each other, we could say that contact is "broken," or the switch has been "opened.")... 

When putting away the components until the next lab session, fold each clip lead in half, and then line them all up next to each other, in parallel, with the folded parts together at the top. Place a rubber band or short wire twist around this group of clip leads. Put the battery in a small, electrically insulating plastic or paper bag. 

Following the diagram of Fig. 2.4, hook up the same 9 volt battery as in the previous chapter, with a black or green clip lead as the negative wire. For the... 

Carefully connect a red clip lead from the positive battery terminal to the red wire of the meter. The pointer will probably move a little bit past the lower black number "5." The black letters to the right read "DC V mA," which means that the lower black scale is used for direct current, either volts or milliamperes. 

Now touch the probes firmly to the primary wires of the transformer, or use clip leads. It will be difficult to get accurate readings with an inexpensive meter, since the red ohms scale is highly compressed, and the red number "one" indicates 1,000 ohms. The small red marks are proportional, and about 200 ohms should be indicated. 

Looking at Fig. 3.2 on the previous page, a black clip lead can be attached to the negative battery terminal, using the careful method of Fig. 1.1 on page 2. Then "bottom" wires of the two resistors (brown-black-red and red-red-red as on page 29) can be crossed, and the other end of the black clip lead is attached to both of them at the same time, as in Fig. 3.3 below. 

Fig. 3.3 Attaching a clip lead to two wires at the same time.

A red clip lead is then attached to the positive terminal of the 9V battery and also to the red metal "probe" of the multimeter. The rotary switch of the meter is set to "150mA DC" (or a similar range if a different kind of meter is used). The black probe of the meter is touched firmly to the other ("upper") end of the 1,000 ohm resistor. On the zero to 15 scale (which now really... 

When the black probe of the meter is then touched to the upper lead of the 2.2K resistor, the current is only 4 ma, as predicted by equation 3.1. When the upper leads of both resistors are bent to be in contact with each other, and the black meter probe is touched to both of them simultaneously (possibly using an additional clip lead), the current is 13 ma, or something similar to that value. 

Connect the 100 Q and 330 Q resistors together with a white clip lead. Although it is not shown specifically in Fig. 4.2, use the multimeter as an ohmmeter to measure the resistance of this pair of resistors in series, either by attaching black and yellow clip leads to the multimeter and then to the bottom and top ends of the pair (black and yellow wires in Fig. 4.2, but no 9V battery is used), or else by simply touching the probes directly to the top and bottom resistor leads, without bothering with the actual black and yellow wires. The total resistance reading should be about 430 Q. In other words,... 

Set the multimeter to "OFF." Looking at Fig. 4.2, the black clip lead should be attached to the battery first, and the other end of the wire is clipped onto the 330 ohm resistor. The top end of that resistor is attached to a white clip lead, and the other end of that lead is clipped onto the 100 ohm resistor. No voltmeters are attached at this time, but the multimeter is set for "150mA DC" (if it is the Radio Shack model), and its black probe is hooked up with a yellow clip lead to the top end of the 100 ohm resistor. 

The last connection to be made (and the first thing to be disconnected after the experiment is finished) is the red meter probe, touched firmly to the positive (smaller) battery terminal (or, alternatively, to a red clip lead attached to the battery). The meter should read about 22 ma. (Disconnect the battery immediately, to avoid overheating.) Using Ohm s law, it is apparent that 9.5V / 0.022 amperes = 430 ohms, which again is the sum of the resistances. 

A green clip lead can now be attached for a moment where the dashed line is on the diagram. The current increased, to 9.5V / 100 ohms = 95 ma, because one of the resistances has been bypassed. The green wire, which was used to bypass or "short circuit" the 330 ohm resistor, is sometimes referred to in electronics as a "jumper cable." It is now completely removed from the circuit. The meter is also removed and set to the "OFF" position. 

The next thing to do is set the meter to measure volts (dc, of course), in the 15 volt range. In order to put the voltmeter in the "2nd" position of Fig. 4.2, it is disconnected, and then its black probe is clipped to the junction between the two resistors, possibly by sharing an alligator clip on the white wire (Fig. 3.3, page 32). Its red probe is attached to the top end of the 100 ohm resistor, possibly by sharing an alligator on a red clip lead. Thus the voltmeter is said to be "connected across" that resistor. 

When the red clip lead is attached from the battery to the 100 ohms (and the voltmeter), this situation will be analogous to Example A, at least in a rough approximation. The difference between Glass 1 and Glass 2 is like the voltmeter reading. The 100 Q is like Valve 1, and the 330 Q is like Valve 2. There is not much voltage, about 2.2 volts (compared to the battery s 9.5 volts), just like there was not much difference in pressures in Example A. 

Another symbol for a rheostat, equivalent to the pot with only two connections in Fig 4.4, is shown below in Fig. 4.5. The open circles are "terminals," which can be connected by any practical means, such as plugs, clip leads, etc. 

Battery, nine volts Clip leads, 14 inch, one set. Resistor, 100 ohm, 10 or 1 watt Resistor, 330 ohm, 1/2 or 1/4 watt Potentiometer, 5K Multimeter (or "multitester")... 

It can be seen from the experiment that "switching" the 330 in and out of the circuit does affect the 2.2K, which it is not supposed to do. If a clip lead (green if available) is put where the short-dash line is shown, then there is hardly any effect, which is the desired result of having a "good ground."... 

If it was a DPDT type (double pole, double throw), then there would be two switches hooked together mechanically, so two movable parts would simultaneously make contact with two upper terminals, or else with two lower terminals. Thus there would be 6 terminals altogether. At any rate, we will use no real switch at all, just a loose clip lead. 

As usual in this course, there is no real switch, and a clip lead is simply touched for a short time ("momentarily") to a wire or terminal, to put a short burst of current through half of the "12 volt" coil (ordinarily the secondary, but acting as a primary here). When the current is suddenly increased, a downward spike appears on the scope, showing that the "top" half of the center-tapped coil has... 

The experimenter should construct the circuit of Fig. 10,5 on the next page. At first, the wires shown as dashed lines are not connected. The experiment is done as on page 7 of the first chapter, where the "switch" is opened suddenly, and there is a bright flash in the neon bulb. Then the short circuiting clip lead (dashed line in the diagram) is attached to the transformer s 120 volt coil (heavily insulated black wires or soldered-on power cord). Now when the switch is opened, there is no neon flash. What is happening is that current is induced in the "shorted" coil, as the battery current suddenly decreases. This new current has a magnetic field... 

---