|MadSci Net: General Biology (View this file without Frames)|
That is a very good question... and very humane, too. It certainly shows that you have the makings of a good scientist since all scientists should be as concerned about their subjects as you. In order to help ensure the safety of your test subject, however, I'll take the scenic route to your answer. But, in the interest of satisfying the more impatient among us, suffice to say that you (and the planarian) are quite safe using any of the 3 batteries you suggested (the 3, 6, or 9 Volt).
You can use any of the three batteries because you need a relatively high voltage in order to really start damaging living cells... something on the order of at least 500V (please refer to Otto Glaser's Medical Physics; specifically, p. 344). The reason for this is that usually it's the current passing through the organism that causes the real damage. Since the resistance of most cells is so high, the current induced from a small battery (like the 9V one) is too tiny to do any lasting damage. If you'll allow me a slight digression into physics, the relationship between current (I), voltage (V), and resistance (R) is given by Ohm's Law: V = IR. Thus, for our case, the current is related to the voltage by I = V/R and, since the resistance (R) is so high, the induced current ends up being very small. If you're interested in learning more about the physics behind this, I highly recommend Larry Gonick's Cartoon Guide to Physics as a very entertaining and informative start. Safety note: the 120V wall outlet and non-linear skin resistance of humans CAN lead to lethal consequences... this discussion is strictly based on direct current studies of cells in vitro only; in other words, it is definitely NOT a good idea to stick anything besides plugs into wall outlets :).
Okay, although we now know that the planaria are going to be safe from significant electrical harm, I'd like to offer a few suggestions to help make your experiment go a little smoother. Almost all of these suggestions are from this great site on Learning and Memory in Planaria. The optimal equipment, as recommended by this web site, is a 6V lantern battery used with some low-voltage wire (bell wire). The response of the planaria was best for this combination rather than the AA batteries and regular wire. Finally, be sure to use pond water or artificial pond water, instead of water from the tap, to store the planaria (regular tap water may contain chemicals and ions that could kill the planaria). Well, I hope this helps with your project; if you have any further questions about any of this, please don't hesitate to drop me a line.
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