# Making an E&M desktop experiment kit

Desktop experiments in electrostatics, circuits, and magnetism are integrated with theory in Volume 2 (Electric & Magnetic Interactions). Whether these experiments are done in a formal lab setting or in a lecture hall, they provide important experience in reasoning about phenomena.

Desktop experiment kits are conveniently available from PASCO (item EM-8675). If however you prefer to assemble your own kits, we offer detailed procurement information gleaned from an earlier period when we assembled our own kits. We recommend buying kits, because they are of high quality, and it is more difficult than you might imagine to procure all of the pieces you need.

Some of this data may be invalid by now. If you find a better source for any of these components, please let Ruth Chabay or Bruce Sherwood know, and we’ll post it here.

An experiment kit contains the following:
1 roll of “invisible” tape
2 D-cell batteries
1 battery holder
2 #14 flashlight bulbs
2 #48 flashlight bulbs
2 flashlight bulb sockets
7 jumpers with alligator clips
2 18-inch (45-cm) lengths of Nichrome wire, #26 and #30 gauge
1 magnetic compass
1 6-foot (about 2-meter) length of hookup wire
1 bar magnet
1 takeout Chinese-food box (4 in. by 3.5 in. by 4 in.) to hold the equipment

One scheme is to issue each student a kit and charge a lab fee to cover the cost, or rent the kit for a semester. In other institutional settings one might choose to have kits available in a formal lab setting and not have individual ownership. One also needs a small supply of aluminum foil and thread for electrostatics experiments, and unmagnetized iron nails (3 per student) for some magnetism experiments.

###### Invisible tape

For the electrostatics experiments we use a cheap local drugstore brand of invisible tape. This tape is manufactured by LePage. We got 500-inch tape, a half inch wide, on sale for 50 cents. Upper (U) Rite-Aid tape is charged negatively, whereas with Scotch brand Magic(TM) Tape a U tape is charged positively. Each lab period typically consumes half a dozen 20-cm (8-in) lengths of tape, so one tape is sufficient for about 10 lab periods.

###### Bulb sockets

We ordered bulb sockets from Science Kit & Boreal Laboratories, 777 East Park Drive, Tonawanda NY 14160-6781 (phone 716-874-6020 or 213-944-6317). A package of 15 lamp sockets #66742 is $8.50. On one occasion we bought lamp sockets for$0.25 each from Mouser Electronics, 2401 Highway 287 North, Mansfield TX 76063, catalog number 35LH010. These sockets were less satisfactory, because their shorter ears led to occasional shorting between ear and body if the clip lead rotated on the ear to which it was attached.

###### Jumpers

Originally we bought some of the mini-alligator clips (the kind with teeth) from Radio Shack (package of 12 for $1.70), but when they couldn’t supply enough of them we got the rest from a local electronics distributor for a slightly higher price. The latter were copper and more satisfactory than the steel Radio Shack clips, because the steel clips can interfere with the compass and the copper clips hold better and make better contact. We used braided hookup wire from Radio Shack, in several colors, and attached clips by soldering. Each kit contained six jumpers that were 10 inches long (including the length of the clips) and one longer jumper that was 18 inches long. In the second offering of the course we bought already-assembled jumpers in packages of ten 14-inch leads for$1.95 (catalog ALCP) from Jameco Electronics, 1355 Shoreway Road, Belmont CA 94002. These were more satisfactory in that the alligator clip was covered by a plastic sleeve which prevented tangling of the jumpers in the box. A disadvantage is that the wire is only crimped to the clip, and not particularly well, so that occasionally a clip lead may be open or intermittently open underneath the plastic sleeve, where you might not think to look. The jumpers have steel clips, which can interfere with the compass readings. We included seven of these jumpers per kit.

###### Nichrome wire

For ohmic resistors we include 18-inch lengths of #26 and #30 Nichrome wire, whose cross-sectional areas differ by about a factor of 2.5 (it is difficult to find wires with an area ratio of 2). We ordered Nichrome wire from VWR Scientific, 4717 Hinckley Industrial Parkway, Cleveland OH 44109 (800-252-1234).

###### Magnetic compass

Originally we used inexpensive small air-filled compasses, but they turned out to be very unsatisfactory for our purposes. The needle would often hang up on the pivot, and give erroneous and misleading results which sometimes induced important misconceptions. Moreover, their physical small size made them hard to work with. The most serious consideration is that the glass should be near enough to the needle that a wire carrying 0.3 ampere along the top of the compass should give about a 40-degree deflection of the needle. It is unfortunate that most compasses are built with the glass cover rather far from the needle.

###### Capacitor

Digi-Key in Minnesota (800-344-4539) according to their web-based catalog (www.digikey.com) carries Panasonic AL series gold capacitors, electric double layer capacitors, that are usable in desktop experiments. In particular, Digi-Key part number P6963-ND has 1.0 farad and low “effective internal resistance” (1 ohm), which are the key issues. The Panasonic part number is EEC-A0EL105. The rated working voltage is only 2.5 volts, but this seems to be a conservative rating, and we find that it works with two ordinary D cells, whose combined voltage is actually less than 3 volts. We have been told that this is not good practice, but apparently for the small, short-time use we make of the capacitors there isn’t a problem. The last time we checked, the price of the Panasonic capacitors was only $4.15 each in lots of one,$2.79 each in lots of 100. In our own teaching we originally used 0.47 farad capacitors, but apparently Panasonic has stopped making this size. Our textbook now refers only to 1 farad capacitors.