the following detailed description is of the best presently contemplated modes of carrying out the invention . this description is not to be taken in a limiting sense , but is made merely for the purpose of illustrating general principles of embodiments of the invention . the scope of the invention is best defined by the appended claims . referring first to fig4 fig4 illustrates a pda pen 50 that includes a pen tube processed according to the method illustrated in fig1 and 2 hereinbelow . the pda pen 50 has a pen tube 51 , a cap 57 and an led stylus 59 . the led stylus 59 can produce light to illuminate the touch control screen of a pda . the interior wall of the front end of the pen tube 51 has a metal layer 52 that electrically connects a pcb 53 ( acting as a control circuit ) to control the illumination of the led 59 . at least one battery 54 ( fig4 shows two batteries 54 ) is retained inside the tube 51 , with the batteries 54 having a front end that is electrically coupled to the pcb 53 . the rear end of pen tube 51 retains a spring 55 and a conductive block 56 that are positioned between the batteries 54 and the cap 57 . the cap 57 is fitted at the rear end of the pen tube 51 . the bias of the spring 55 against the block 56 and the batteries 54 function to retain the batteries 54 firmly in place inside the pen tube 51 to maintain the necessary electrical connection . the cap 57 also has an external protruding gripping block 58 which can be used by a user to push the pda pen 50 out of the groove of a pda . although fig4 provides a very simple and general description of an illuminating pda pen 50 , the pda pen 50 can have a structure and operation that is very similar to the pda pens shown and described in co - pending application ser . no . 10 / 039 , 696 , filed jan . 4 , 2002 , entitled “ touch control pen for pda ”, whose disclosure is incorporated by this reference as though set forth fully herein . [ 0024 ] fig1 and 2 illustrate a method of producing a pen tube 10 ( which is the same as pen tube 51 described above ) according to the present invention in which the rear portion of the tube 10 is isolated to avoid electrical shorts , yet which has a sufficiently small diameter so that the tube 10 can be fitted inside the groove of any conventional pda . the method illustrated in fig1 and 2 involves anodizing a portion of the tube 10 . first , a base plate 20 is provided for processing pen tubes 10 . each base plate 20 has a plurality of identical plugs 22 , 24 , 26 . each plug ( e . g ., 22 ) itself defines a lower primary plug 224 and an upper secondary plug 222 , with the secondary plug 222 having a smaller diameter than the primary plug 224 . the plug 24 also has a primary plug 244 and a smaller - diameter secondary plug 242 . next , each tube 10 is fitted on to a separate plug 22 , 24 , 26 by placing one open end of the tube 10 over the secondary plug 222 and 242 of a corresponding plug 22 , 24 or 26 . a metal layer ( e . g ., aluminum ) is then coated on to the internal surface of the tube 10 . the area of the internal surface of each tube 10 that is occupied by a corresponding secondary plug 222 , 242 defines an occupied area 15 on the internal surface at one end of the tube 10 . thus , the size of the occupied area 15 is determined by the size and shape of the secondary plug 222 or 242 . next , as shown in fig2 the base plate 20 and the tubes 10 that have been fitted over the plugs 22 , 24 , 26 are turned sideways ( e . g ., by tilting or turning the base plate 20 by ninety degrees ) and placed into an electrolyte solution 32 of an electrobath 30 for anodized treatment . since each occupied area 15 is being blocked by a corresponding secondary plug 222 , 242 , the occupied areas 15 are not exposed to electrolyte 32 , so that an oxide ( e . g ., aluminum oxide ) is not formed on these occupied areas 15 , thereby still leaving a conductive metal layer at the occupied areas 15 . the metal on the other portions of the internal surface that are not occupied or blocked by the secondary plugs 222 , 242 will react with the electrolyte 32 to produce an oxide ( e . g ., aluminum oxide ) that functions as an isolative layer . as a non - limiting example , aluminum oxide is a ceramic material having features of isolation and hardness . in other words , the metal layer not occupied by a secondary plug 222 , 242 in tube 10 is transformed into an oxide layer 40 ( see fig3 ) for isolative function , while the portion of the metal layer occupied by a secondary plug 222 , 242 remains a conductor 45 . fig3 illustrates the pen tube 10 after being processed in the manner described and illustrated in connection with fig1 and 2 . referring back to fig4 the conductive portion 45 of the tube 10 is the location where the pcb 53 is positioned . thus , the conductive portion 53 can electrically connect the pcb 53 with the led stylus 59 , while having the rest of the tube 10 be electrically isolated to avoid electrical shorts . thus , by utilizing the original metal layer on the internal surface of a tube 10 and isolating a selected portion of this metal layer prior to an oxidation treatment , the present invention can accomplish the objective of isolating a portion of the tube 10 to avoid electrical shorts , yet still providing the tube 10 with a diameter that is small enough to allow the tube 10 to be fitted inside the groove of any conventional pda . while the description above refers to particular embodiments of the present invention , it will be understood that many modifications may be made without departing from the spirit thereof . the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention .