Abstract: A yarn tension sensor is disclosed which is adapted for use with a textile yarn processing machine. The sensor includes a spring arm which is contacted by a running yarn, and such that the tension in the yarn acts to deflect the arm. In order to render the deflection of the arm relatively insensitive to external or machine vibrations, while permitting an accurate response to fluctuations in yarn tension, there is provided a structure for damping the spring arm, which includes a magnetic fluid having a portion of the spring arm immersed therein.