A Safe and inexpensive Device To Show the Conductivity of Solutions The standard commercially available device to show the conductivity of solutions in a qualitative manner uses a light bulh connected in series with electrodes and 110-V ac. This device works well, showing, for example, that tap water conduets electricity while distilled water does not. However, I always approach doing this demonstration with trepidation since the electrodes are hare and the possibility for shock is great. I also hesitate showing this dangerous device to students. There is a commercially available device that runs off a 9-V battery, but its output device is a small bulb that is difficult to see a t a distance. The conductivity demonstrator described here is completely safe since it uses an isolated low-voltage ac power supply and a 12-V 15-Wlight bulb that can be purchased from suppliers torecreation vehicles or house trailen. The bulb fits into a porcelain socket mounted on a 4-in. diameter or 4-in. square piece of %in. wood. The socket is fastened with wood screws. The electrodes are made of stainless-steel sheet metal, and are 3 in. long and 2 in. wide with a l-in. right angle tab that is groaved so that the electrode spacing is sdjustable. Securing bolts for the electrodes pass through the openings in the porcelain socket. The electrodes are fastened between washers. The electrodes and bulh are connected in series. The electrode spacing should be of the order of 'Ilsin. or about 1.5 mm for the bulb to be off in distilled water and give a low glow in tap water. Of course, electrolytic solutions cause it to glow brightly. Low-voltage ac transformers can be purchased from asupplier like PartsExpress (340 E. First St., Dayton, OH 45402). A 12-Vac 4.5-A adaptor costs $7.50, and 14-V ac 0.8-A or 16-V ac 1.1-A adaptors cost $3.95. All adaptors worked well, but the higher powered unit is preferred. None short to ground and the assembly can simply be tested by forcing the electrodes to touch. I t is not practical to use higher wattage 12-V bulbs than 15W since their resistance is so low that to get appreciable brightness in tap water the electrodes must he so close together that it is difficult to use the device.
Rubln Baltlno Wright State University Dayion, OH 45435
Volume 68
Number 1 January 1991
79
