Abstract: Electronic equipment having a temperature control air flow passage. A length of filter material has a length portion extending across the upstream end of the flow passage, the filter material being movable longitudinally to replace the clogged length portion with unused filter material. A filter advancement device is operable by a control device actuated when an undesirably low fluid flow rate is achieved through the length portion so as to effect filter replacement. Roller devices give up unused filter material and take up used material. A stepper motor may be used to advance filter material for replacement purposes. The control device is preferably operated upon drop in rotational speed of a fan located immediately downstream of the filter length portion, the speed drop being caused by drag as the filter becomes clogged. Alternatively, the control device comprises a switch which is operated as the filter length portion flexes it becomes clogged, or temperature sensors located at chosen sites.

Abstract: A gage block is designed as a stepped sine block to produce commonly used angles when used with a conventional sine bar without the use of conventional gage blocks. Reference surfaces parallel to one or more bases are provided to produce the desired number of commonly used angles.