reference will now be made in detail to the preferred embodiments of the present application . it can be appreciated that various modification in many different ways can be made to the following illustrative embodiments of the present invention without departing the principle of the present invention disclosed herein . therefore , the scope of the present invention will in no way be limited to the special embodiments in the following . the device of the present invention is adapted to be used in the handwriting pen apparatus for inputting data to a computer . the present embodiment employs an device as shown in fig1 , which comprises a pen body 1 , a circuit board 2 in the pen body 1 , a pen tip 3 in the front end of the pen body , an inductive coil 4 and a dielectric structure 5 . a contact probe 51 is integrated with the pen tip 3 as a whole . the inductive coil 4 is disposed between the contact probe 51 and the pen tip 3 . fig2 is a sectional view along the line a - a of the device shown in fig1 . fig3 is a longitudinal view showing an initial state of the dielectric structure of the apparatus in fig1 . the dielectric structure 5 comprises the contact probe 51 , an elastic conductive pad 52 , a gasket 53 , a first dielectric 54 , a second dielectric 55 , a first electrode 56 , a second electrode 57 , an isolation layer 58 and a housing 59 . the end face of the contact probe 51 has a convex shape which is higher in the middle portion and lower in the periphery . specifically , it can be a conical shape or a shape of spherical cap . upon receiving a force , the contact probe 51 directly acts on one end face of the elastic conductive pad 52 . the elastic conductive pad 52 is made of an elastic conductive material , which deforms upon receiving an external force and restores to its original shape after the external force is withdrawn . on the other end face of the elastic conductive pad 52 are arranged the first dielectric 54 and the second dielectric 55 in parallel , which are made of material of relatively high dielectric constant . the gasket 53 made of material of low dielectric constant is disposed between the two dielectrics and the elastic conductive pad 52 . in a preferred embodiment , the gasket 53 is configured to be an annular body . obviously , the gasket 53 may take the form of two gaskets separate from each other . the first dielectric 54 and the second dielectric 55 are separated from each other to form the isolation layer 58 , which has very low dielectric constant and may be formed naturally by air or may be filled with material of low dielectric constant . the first dielectric 54 and the second dielectric 55 are symmetrical in shape , disposed in such a way that they form a mirror image to each other , for example , both are of a semi - circular shape and face each other while centering on a center of a circular shape . that is to say , the first and second dielectrics plus the isolation layer 58 constitute a complete circle . the first and second dielectrics 54 and 55 are semi - circular or substantially semi - circular in shape in the present embodiment . the other end face of the first and second dielectrics 54 and 55 are connected to the first electrode 56 and the second electrode 57 respectively . as shown in fig2 , the shape of the first electrode 56 and the second electrode 57 are substantially identical with or correspond to that of the first dielectric 54 and the second dielectric 55 respectively . further , the first electrode 56 and the second electrode 57 are tightly attached to or abut against an end face of the first dielectric 54 and the second dielectric 55 , respectively . lead wire or conductor is led out from the first electrode 56 and the second electrode 57 . in one application of the present invention , as shown in fig8 , the first electrode 56 and the second electrode 57 of the dielectric structure 5 are further connected to a resonant circuit , which can generate resonance in response to an oscillation signal of a certain frequency and receive a resonant signal by a receiving circuit . the resonant circuit can sense the variation of the pressure by receiving an associated resonant signal . specifically , if the first electrode 56 and the second electrode 57 are connected to a capacitance - inductance or resistance - capacitance oscillating circuit or other similar oscillating circuit , a frequency value and associated characteristics can be obtained by varying the oscillating frequency via the variation of the capacitance . based on this variation of the frequency parameter , a feedback to the variation of electrode capacitance on the dielectrics , i . e . a status of force on the pen tip 3 , is obtained . further , in order to ensure the first dielectric 54 , the second dielectric 55 and the isolation layer 58 combine with each other well , a housing 59 is provided at outside thereof for reinforcing and stabilizing purpose , so that tight combination between them is guaranteed . next , the operation of the present embodiment will be explained in the following . since the contact probe 51 and the pen tip 3 are integrated as a whole , they always are in the same status . the initial state is assumed as a state in which the pen tip 3 is not subject to an external force . then as shown in fig1 , the pen tip 3 , the contact probe 51 and the elastic conductive pad 52 are all in an initial state , i . e . not under effect of any external force . at this time , the isolation layer 58 makes the first dielectric 54 and the second dielectric 55 separate from each other , and the capacitance between the first electrode 56 and the second electrode 57 can be neglected , i . e . has an initial value of zero . at the time of initial value of zero , the compression between the elastic conductive pad and the end face of the dielectrics is shown in fig5 . fig4 is a longitudinal sectional view showing the dielectric structure of the apparatus in fig1 in a status under effect of an external force . when the pen tip 3 is subject to an external force , that is , when handwriting by pen is conducted on a desk surface or other plane , an action of force , which is varying , is generated . the force and the variation thereof are transmitted to the contact probe 51 by the pen tip 3 and further acts on the elastic conductive pad 52 ; the elastic conductive pad 52 then deforms and acts on the end face of the first dielectric 54 and the second dielectric 55 . the elastic conductive pad 52 is a good electrical conductor , which makes the first dielectric 54 and the second dielectric 55 be previously independent of each other become an associated whole body , that is , two dielectrics are communicated with each other and produce a capacitance between the first electrode 56 and the second electrode 57 . fig6 is a sectional view showing a state in which the end face of the dielectrics and the elastic conductive pad are subject to an external force . as shown in fig4 , when the elastic conductive pad 52 begins to compress the end faces of the first dielectric 54 and the second dielectric 55 , a minimum capacitance is generated between the first electrode 56 and the second electrode 57 . the greater the force on the contact probe 51 , the greater the deformation of the elastic conductive pad 52 , and therefore , the greater the compression area at which the elastic conductive pad 52 compresses the end faces of the first dielectric 54 and the second dielectric 55 . the area of the hatched portion in fig6 represents the dimension of the compression area . the capacitance between the first electrode 56 and the second electrode 57 gets greater along with the variation of the compression area until reaching a maximum value . also , this capacitance value varies with the variation of the external force . the above process is reversible . that is , when the external force is reduced , the deformation of the elastic conductive pad 52 gets small , and the compression area between the elastic conductive pad 52 and the end faces of the first and second dielectrics 54 and 55 gets smaller correspondingly , and therefore , the capacitance value between the first electrode 56 and the second electrode 57 gets less until reaching a minimum value . when the external force is withdrawn , the elastic conductive pad 52 returns to its original state due to its elasticity . since the elastic conductive pad 52 returns to its initial state relative to the first dielectric 54 and the second dielectric 55 , the capacitance between the first electrode 56 and the second electrode 57 restores to the initial value of zero . fig7 is the front view of further embodiment of the dielectric structure 5 according to the present invention . in order to reinforce and ensure the elasticity effect of the elastic conductive pad 52 , an elastic conductor 522 is further provided between the contact probe 51 and the elastic conductive pad 52 . more preferably , an elastic conductor 533 is further provided between the elastic conductive pad 52 and the gasket 53 . the above process can be conducted repeatedly , so that the value and variation of the capacitance between the first electrode 56 and the second electrode 57 can be measured and the mutual relationship therebetween along with variation of the external force can be obtained . for the convenience of sensing the variation of the external force , it is desirable that the deformation generated when the contact probe 51 acts on the elastic conductive pad 52 tends to be more linear . for this purpose , the end face of the contact probe 51 is designed to be a convex shape which is high in the middle and low in the periphery , for example , to be a conical - like shape or a shape like spherical cap . now , referring to fig8 , detailed description will be made to the structure and operation principle of the dielectric structure 5 according to an example of the application of the dielectric structure 5 of the present invention . it is to be noted that , the example disclosed herein is just a preferred embodiment of the application of the dielectric structure 5 according to the present invention . as will be appreciated by those skilled in the art , the device for varying capacitance according to the present invention , i . e . the dielectric structure 5 may be applied to any device and system in which the capacitance variation is necessary . the following preferred embodiment shall not be interpreted as for limiting the protection scope of the present invention . as is known by those skilled in the art , an active - oscillation transmitter coil can generate stable oscillation signal , such as a sine wave , a sawtooth wave , or the like . the dielectric structure 5 of the present invention can be regarded as a variable capacitor . as shown in fig8 , the dielectric structure 5 can be connected to a fixed capacitor c 1 and an induction coil 4 in parallel . therefore , the induction coil 4 and the dielectric structure 5 constitute a simple resonant circuit . as long as the inducted inductance and the capacitance are properly selected , the induction coil 4 can generate resonance to the oscillation signal generated by the oscillation transmitter coil 60 within an effective distance . at the same time , if an oscillation receiving coil 61 exists in the effective distance , the resonance signal can be sensed by it . further , when the dielectric structure 5 , i . e . the value of the variable capacitance varies , the oscillation frequency of the resonant circuit will also varies in proportion to the variation of the value of the capacitance . if an additional receiving coil ( not shown ) corresponding to the oscillation receiving coil 61 is further provided within the effective distance , the resonant signal can be received . in the forgoing description , the relation between the variation of the capacitance of the dielectric structure 5 and the external force applied on the dielectric structure have been described . by applying the above dielectric structure 5 in the circuit configuration shown in fig8 and making use of the relation between the variation of the capacitance and the associated external force , a complete circuit and a system for sensing the status of an external force can be obtained and used for a handwriting pen for inputting data to a computer by handwriting . the device for varying capacitance of the present invention has a simple structure and a high sensitivity , and therefore , has high value for application . although preferred embodiments of the present application have been shown and described above , it is not intended to limit the scope , application and configuration of the present invention . instead , it would be appreciated that , the scope of the present invention is defined only by the appended claims , and that some suitable modifications and variations can be made to the details of the present invention without departing from the spirit and scope of the present invention .