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The movement of a charged body, such as a charged human body or a charged device, often causes a malfunction or failure in an electronic apparatus because an induced voltage is generated in a partially opened metal box of the apparatus. We cannot ignore this type of electronic apparatus malfunction when the relative humidity in a room decreases because of the use of an air conditioner. We need to design the electronic apparatus to safely prevent such malfunctions. The induced voltage generated in the metal box must be measured for the safe design of an electronic apparatus. We measure the induced voltage generated in the metal box without electrical connections using a spark gap and an electromagnetic wave sensor. In this paper, we study the relationship between the induced voltages and the volume of the metal boxes. Five metal boxes with different depths are used to vary the volume of the boxes in the experiment. The measured result shows that the induced voltage generated in the metal box is independent of the volume of the metal boxes (the depth of the boxes). Thus, the value of the induced voltage generated in a small metal box is almost the same as that in a big metal box. Therefore, an induced voltage can be generated in a small metal box, such as a tablet device. The result of this study is useful for the safe design of an electronic apparatus to prevent malfunctions and failure of the apparatus.