How can I apply extremely low forces to an object?

[StrawberrySaturn]

I want to be able to build or find a mechanical device that can apply very low force/pressure to a material. I have considered using a stepping motor with a flat needle tip but I can't think of a way to make this delicate enough to not destroy my sample. I am wanting to apply forces in the range of milli to micro Newtons.
I know that this can be done with nanoindentation machinery but I want something smaller/semi-portable that can apply a force and doesn't need to do anything else.
Any tips would be appreciated.

 

[Tom.G]

An old style analog panel meter. Take off the faceplate and you have access to the moving needle. It's easy to control the force my varying the applied voltage.

 

[StrawberrySaturn]

An old style analog panel meter. Take off the faceplate and you have access to the moving needle. It's easy to control the force my varying the applied voltage.

This is a really interesting idea! Do you think I would be able to know the exact force I was exerting?

 

[Baluncore]

This is a really interesting idea! Do you think I would be able to know the exact force I was exerting?

Yes. The equations for force with a moving coil meter are well defined. The force is directly proportional to the meter current. You could calibrate the movement at maximum current then use one thousandth of that to apply the force. Avoid meter voltage because it is current that decides force and the resistance of the copper wire and hairsprings are temperature dependent.

Because the two hairsprings return the meter to zero you should operate the meter at that zero deflection. I guess you will attach a miniature tool at right angles to the tip of the pointer. It is then necessary to re-adjust the movement for static balance so that zero is independent of orientation. Think of the meter pointer as a lever. The longer the pointer the less the force at the tip.

 

[Tom.G]

Oh! You want to know the actual force?

You could always calibrate with known weights. It's probably easier than you think.

For instance knitting yarn is sold in a variety of weights with both the weight and length given. A common medium gauge ball of wool would be 100grams/200meters; two meters per gram; 1mG would be 2mm. The yarn comes as a single strand, two, three, four, or six strand, with three strand more common around here, so for greater resolution you can use one strand of a multi-strand yarn. There is also cotton cord and sewing thread available in large spools. I don't know if they are marked with both weight and length so they may take a little more work to get weight per unit length. Use either cotton or wool fibers for this, the static charge from synthetics will drive you crazy.

Other tips.
You need a draft free, rather clean, work area once the meter movement is opened. Both the internal clearances and available torque are tiny.
You may want some optical magnification when working on the meter. I use a pair of cheap reading glasse from the local drug store. I just put them on over my everyday tri-focals!
Two pair of fine tweezers are also handy, non-magnetic only.