Physics 1252 Appendix A: Equipment

SPARK TIMER

The spark timer is a device used for timing the motion of objects when one wishes to do better than just getting average velocities and accelerations as obtained using a stopwatch. The spark timer is an electrical analog to the characters in the children's story who dropped morsels of bread every few seconds while hiking in a forest: their motion could have been determined by keeping track of the positions of the pieces of bread. Using a more mathematical language, the spark-timer allows us to obtain values of the function x(t) where 'x' is a position coordinate at very many values of t, where 't' is the elapsed

time or "clock reading". Question for the reader: do you think one can truly measure instantaneous velocities using this approach?

A clever but simple circuit inside the spark-timer produces high voltage pulses at fixed time intervals: every 1/10, 1/20, 1/30, or 1/60 of a second depending on the device's setting. This high voltage pulse can be used to produce sparks between a moving object and a stationary electrode, or in some cases, between two stationary electrodes. If the sparks go through certain types of paper, a small but visible spot is produced. The position of each spot on the paper is used to determine a moving object's position as a function of time.

Note that the spark-timer must be connected to two conductors in order to function properly. The device's "live wire" or high-voltage output line is heavily insulated and is yellow on most of our devices. It is permanently attached to the spark timer. The spark-timer also features a black connector used to link a second conductor to ground (zero voltage). The live wire should always be connected to the smallest sized conductor consistent with a measurement, and this conductor must be electrically insulated from ground. When the conductor is pulsed to a high voltage, current will "want to flow" to ground through the easiest path possible. It's up to you to set up your spark-timer so you obtain a spark at the desired position. Avoid becoming the "easiest path possible" by never touching nor coming too close (less than a couple inches) to the "live" conductor.

DO NOT TOUCH ANYTHING METAL ON THE APPARATUS WHILE THE SPARK TIMER IS IN OPERATION! It operates at 10,000 volts and can give you a nasty shock. Also be aware of any metal rings, chains, necklaces or watches that you are wearing. If they get too close to the apparatus, the spark may jump to you!!

Last words of wisdom: if you have any questions or doubts about using the spark-timer, consult with your Lab Instructor.

THE DIGITAL MULTIMETER (DMM)

The DMM is a common piece of lab equipment that can be used to measure various electrical quantities, most often current, resistance, and potential. The DMMs you will be using are capable of measuring both "direct current" (DC) and "alternating current" (AC) circuits. Be careful about knowing which type of measurement you need to make, then set your DMM accordingly.

The DMM can measure currents anywhere from 10 amps to a microamp (10-6 amps). This versatility makes the DMM fragile, because measuring a large current while the DMM is prepared to measure a small one will certainly harm the DMM. For example, measuring a 1 ampere current while the DMM is on the 2 milliamp scale will definitely blow a fuse! If you damage the DMM beyond repair, you will have to finish the lab without the DMM.

Measuring Current:

1. Set the selection dial of the DMM to the highest current measurement setting (10 amps). Insert one wire into the socket labeled '10A' and a second wire into the socket labeled 'COM'.

2. Attach the DMM into the circuit as shown below:

3. If no number appears while the DMM is at the 10 amp setting, move the wire from the 10A socket to the 200mA socket and then turn the selection dial to the 200 milliamp (200m) setting. If there is still no reading, change the dial to the 20 milliamp setting, etc.

4. When you have taken your measurement, return the DMM selection dial to the highest current setting (10 amps) and move the wire back to the 10A socket.

Measuring Voltage:

1. Set the DMM selection dial to read DC volts ( ). Insert one wire into the socket labeled 'V]' and a second wire into the socket labeled 'COM'.

2. Connect the two wires from the DMM to the two points between which you want to measure the voltage, as shown below

3. If no number appears, try a different measurement scale. It is a good idea to use a consistent method of trying different potential scales, for instance starting at the highest voltage scale and working your way down the scales until you get a satisfactory reading.

Measuring Resistance: The element whose resistance you are measuring must be free from all other currents (due to other batteries, power supplies, etc.) for the DMM to work. To measure resistance:

1. Set the DMM selection dial to measure ohms (). Insert one wire into the socket labeled 'V' and a second wire into the socket labeled 'COM'.

2. Make sure that the circuit element whose resistance you wish to measure is free of any currents.

3. Attach the wires across the circuit element, as shown in the example below.

4. If no number appears, try a different measurement scale. Use a logical method that covers all scales, such as beginning at the largest scale
(20 M) and working your way down.



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Comments: <Tom Foster>

Last Updated: Fri,June 6, 1996 12:00 PM EST