Here we report construction of a simple electromagnet with novel polepieces which apply a spatially uniform force to superparamagnetic beads in an optical microscope. The wedge-shaped gap was designed to keep ∂Bx∕∂y constant and B large enough to saturate the bead. We achieved fields of 300–600 mT and constant gradients of 67 T∕m over a sample space of 0.5×4 mm2 in the focal plane of the microscope and 0.05 mm along the microscope optic axis. Within this space the maximum force on a 2.8 μm diameter Dynabead was 12 pN with a spatial variation of approximately 10%. Use of the magnet in a biophysical experiment is illustrated by showing that gliding microtubules propelled by the molecular motor kinesin can be stopped by the force of an attached magnetic bead.
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机译：在这里，我们报告了一个带有新型极靴的简单电磁体的构造，该极靴在光学显微镜中向超顺磁珠施加了空间均匀的力。楔形的间隙设计成使∂Bxyy恒定且B足够大以使磁珠饱和。我们在显微镜的焦平面上的0.5×4 mm2的样品空间内，沿显微镜光轴的0.05 mm的样品空间，实现了300–600 mT的磁场和67 T ∕ m的恒定梯度。在此空间内，直径2.8μm的Dynabead上的最大力为12 pN，空间变化约为10％。通过显示在分子物理驱动蛋白推动下滑动的微管可以通过附着的磁珠的作用力来阻止，从而说明了在生物物理实验中使用磁体的情况。
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