Now suppose you're crouching down in order to move the test masses, with your centre of gravity one metre from the closer test mass, and that you weigh 65 kg. Plugging these numbers into the calculator shows that your own gravitational attraction on the nearer end of the beam is 0.000147 dynes, 1.7 times as great as that of the test mass. Your actual influence on the motion of the balance arm is less, however, since what matters is the difference in force exerted on the masses at the two ends of the balance arm. Since your centre of gravity is more distant than the test masses, the difference is less.
Let's work it out. Assume the centres of gravity of the two masses on the balance arm are 25 cm apart, and that you're crouching so the arm makes a 45° angle with your centre of gravity, one metre from the centre of the arm. The nearer mass is then 17.68 cm closer than the more distant one and the difference in gravitational attraction (or tidal force) on the two masses is the difference in attraction on a mass 91.16 cm distant and one 108.84 cm away. The calculator gives the attraction on the near end of the arm as 0.0001764 dynes and the far end as 0.0001238 dyne, with a difference of 0.0000527 dynes. Now recall that the force exerted by the test mass was 0.000085 dynes, only 1.6 times as large, so even taking into account the reduced tidal influence due to your greater distance, the force you exert on the balance cannot be neglected. This makes it essential to remotely monitor the experiment so your own mass doesn't disrupt it.
In practice, air currents due to your motion and resulting from convection driven by your body's temperature being above room temperature may exert greater forces on the balance arm than the gravitational field generated by your mass. In any case, it's best to let the experiment evolve on its own, observed from elsewhere.
That's what I deserve for not rereading the page after a decade, sigh.
I have an 8-year-old sister who is very interested in science. The school she attends (in rural Indiana) takes the mantra of "teaching to the test" to a whole new level; my sister has already come home from school many days crying and stressed over fear that she won't pass her state's proficiency tests (which are 5+ months away). They are underfunded and the science curriculum is what most of the (overtly religious) teachers are undertrained in, and so science is essentially not addressed. My sister has about an order of magnitude more homework on cursive writing than on anything related to science.
I want to purchase a gift for her for the holidays this year (the occasion doesn't matter, but it's a time when I'll actually be home so I can give her the gift and play with her / explain how to use it / hopefully help her start thinking about some things). I'm willing to consider age-appropriate ideas across several price ranges, but I want to think of something that will deliver a lot of utility: i.e. it should be compelling enough that an 8-year-old will actually like using it and there should be at least some evidence that she will benefit from it.
I've considered things like the EDUbuntu computers that come with lots of educational software, but actually buying one seems to be not straightforward. Is the best thing to buy a cheap netbook at then install EDUbuntu myself? Is a netbook too much for an 8-year-old? I'm mostly focused on things that will help her be proficient with computers and potentially help her develop a more sophisticated interest in them as she grows up. The Lego Mindstorm robotics stuff also crossed my mind.
Does anyone have experience with this / know of resources for making a good investment? Or am I way over-thinking this and just some regular Legos or art supplies are going to do essentially just as much good?
Added 02/25/2012
Much of the advice was very helpful. I ultimately found out that my young sister was interested in "mixing chemicals together because it looked cool." An item that I found which looked like a good way to bridge the gap between her more girly interests and her interest in chemistry was Perfume Science from Thames and Kosmos. I also purchased a variety kit from Snap Circuits. She loved both items and we spent a good bit of time playing with them during my trip home to see my family. We made several different kinds of perfume and also did some activities that helped explain how different extracts have been acquired throughout human history for their smells. Ultimately, my sister made a science fair project (something which surprised my parents a lot) based around one of the activities in the perfume science booklet, and she won second place.
I was very happy with both my decision to ask this question on LessWrong (despite the title of the post, which seems to seethingly annoy many LWers) and my purchase decisions. I used the website Fat Brain Toys to purchase the items and everything arrived without a problem. That site seemed to also have a reasonably good selection for more educationally oriented toys across several ages, and with useful customer reviews.