Arc tube with shortened total length, manufacturing method for arc tube, and low-pressure mercury lamp

1. A manufacturing method for an arc tube formed by turning a glass tube at a substantially middle thereof and winding the glass tube from the middle to both ends thereof around an axis to form a double spiral, and sealing a pair of electrodes at both the ends of the glass tube, the manufacturing method comprising the steps of:

winding the glass tube that is softened by heating, along a groove in a double spiral formed on an outer circumference of a mandrel;

removing the glass tube that is wound in a double spiral from the mandrel;

making a pitch of (a) a spiral part in a vicinity of a sealing part at each end of the glass tube and (b) an adjacent spiral part in a direction of the axis, larger than a pitch of other adjacent spiral parts, to widen a gap between the sealing part and the adjacent spiral part; and

pinch-sealing the electrodes in the sealing parts at both the ends of the glass tube.

2. The manufacturing method of claim 1 , wherein

in the step of making the pitch of the spiral part in the vicinity of the sealing part larger than the pitch of other adjacent spiral parts, a part of the glass tube away from an end face of the sealing part in a winding direction by a predetermined amount of spiral is heated to a temperature that is higher than a softening point of the glass tube and lower than an operating temperature of the glass tube, and the heated part of the glass tube is bent in the direction of the axis so that a gap between the spiral part in the vicinity of the sealing part and the adjacent spiral part widens gradually from the heated part toward the sealing part.

3. The manufacturing method of claim 1 , wherein

in the step of sealing the electrodes, the sealing parts of the glass tube are heated to a temperature that is equal to or lower than a temperature 120° C. higher than an operating temperature of the glass tube, so that the electrodes are sealed in the sealing parts.

4. The manufacturing method of claim 1 , wherein

in the winding step, the glass tube is wound, while a gas whose pressure is controlled is blown into the glass tube.

5. A manufacturing method for an arc tube formed by turning a glass tube at a substantially middle thereof and winding the glass tube from the middle to both ends thereof around an axis to form a double spiral, and sealing a pair of electrodes at both the ends of the glass tube, the manufacturing method comprising the steps of:

winding the glass tube that is softened by heating, along a groove in a double spiral formed on an outer circumference of a mandrel;

removing the glass tube that is wound in a double spiral from the mandrel;

making a pitch of (a) a spiral part in a vicinity of a sealing part at each end of the glass tube and (b) an adjacent spiral part in a direction of the axis, larger than a pitch of other adjacent spiral parts, to widen a gap between the sealing part and the adjacent spiral part, by heating a part of the glass tube away from an end face of the sealing part in a winding direction by a predetermined amount of spiral to a bending temperature that is higher than a softening point of the glass tube and lower than an operating temperature of the glass tube, and bending the heated part of the glass tube in the direction of the axis; and

sealing the electrodes in the sealing parts at both the ends of the glass tube.

6. The manufacturing method of claim 5 , wherein

the bending temperature is equal to or lower than a temperature 120° C. higher than the softening point.

7. The manufacturing method of claim 5 , wherein

in the winding step, the glass tube is wound, while a gas whose pressure is controlled is blown into the glass tube.

8. A manufacturing method for an arc tube formed by turning a glass tube at a substantially middle thereof and winding the glass tube from the middle to both ends thereof around an axis to form a double spiral, and sealing a pair of electrodes at both the ends of the glass tube, the manufacturing method comprising the steps of:

winding the glass tube that is softened by heating, along a groove in a double spiral formed on an outer circumference of a mandrel, while a gas whose pressure is controlled is blown into the glass tube;

removing the glass tube that is wound in a double spiral from the mandrel;

making a pitch of (a) a spiral part in a vicinity of a sealing part at each end of the glass tube and (b) an adjacent spiral part in a direction of the axis, larger than a pitch of other adjacent spiral parts, to widen a gap between the sealing part and the adjacent spiral part; and

sealing the electrodes in the sealing parts at both the ends of the glass tube.