A biodegradable polymer is disclosed having the composition g) from 8 to 80% by weight of a starch modified to include an hydroxyalkyl C2-6 group or modified by reaction with an anhydride of a dicarboxylic acid, preferably hydroxypropylated high amylose starch, a) from 0 to 87. The upper electrode 24 is grounded, the peripheral electrode 26 is powered by a high frequency AC power supply, while the lower electrode 28 is powered by a low frequency AC power supply, as well as a DC power supply. Then, interference between the subject W and an X-ray source 1 is monitored on the basis of the information, or the rotary stage 3 is automatically positioned at a location where the subject approaches most closely to the X-ray source 1 without interference. The cover is capable of being secured to the insulating washer, wherein an insulating volume is created with the cover secured to the washer. A data transfer method that is performed at an information server includes receiving a data request from a proxy server, extracting profile information added to the data request by the proxy server, using the extracted profile information to generate a response, and sending the response to the proxy server. The two chambers have opposing tapered shapes, so that an impact anywhere along them will create a different pressure wave or pulse in each chamber.