JP3421248B2 - Weak ammeter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weak ammeter such as a Faraday cup ammeter for measuring a weak current,
In particular, the present invention relates to a weak ammeter capable of effectively reducing noise that appears in a weak current to be measured.

[0002]

2. Description of the Related Art It is known that in a weak ammeter such as a Faraday cup ammeter, noise appears in a weak current to be measured due to various external or internal causes. Since such noise is the largest factor that hinders accurate measurement, various attempts have been made to reduce the noise.

As one of such conventional attempts, an attempt has been made to cover the entire weak ammeter with a metal box to shield electromagnetic waves incident from the outside.

[0004]

However, in such a conventional attempt, it is not possible to reduce the noise appearing in the weak current to a sufficient level (several fA level). There is a problem that it is impossible to measure a weak current corresponding to the above.

The present invention has been made in view of the above points, and an object thereof is to provide a weak ammeter capable of effectively reducing noise appearing in a weak current to be measured.

[0006]

The present invention comprises a current source for generating a weak current to be measured and an amplifier for amplifying the weak current generated by the current source, wherein the current source has a first end surface. And a rod-shaped first electrode attached to the first end face for guiding a weak current to the outside.
A second end face opposed to the first end face, and a second electrode attached to the second end face and connected to the first electrode to guide a weak current therein, the receiving electrode receiving the rod-shaped first electrode A second electrode as a part, and between the first end face of the current source and the second end face of the amplifier,
There is provided a weak ammeter characterized in that a tubular member surrounding the connection portion of the first electrode and the second electrode is sandwiched so as to shield an electromagnetic wave incident on the connection portion.

In the present invention, the tubular member is
It is preferable to surround the connection portion of the first electrode and the second electrode in a state where stress is applied to the first end surface and the second end surface.

According to the present invention, the first electrode is connected to the second electrode in a state where the tubular member that shields electromagnetic waves is sandwiched between the first end face of the current source and the second end face of the amplifier. ,
It is possible to effectively shield the electromagnetic wave that is incident on the connection portion of the first electrode and the second electrode through which the weak current flows, and thus it is possible to effectively reduce the noise that appears in the weak current to be measured.

Further, according to the present invention, stress is applied to the first end face and the second end face in a state where the tubular member is sandwiched between the first end face of the current source and the second end face of the amplifier. ,
Since the contact state between the first electrode and the second electrode is good, noise that appears in the weak current to be measured can be effectively reduced.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1, 2, and 4 are views showing a first embodiment of a weak ammeter according to the present invention. Here, FIG. 1 and FIG. 2 are Faraday cup ammeters (F
FIG. 4 is a sectional view showing the main part of a CE (Faraday cup electrometer), and FIG. 4 shows the Faraday cup ammeter shown in FIGS. 1 and 2 as a differential electric mobility measuring device (DMA).
It is a cross-sectional view showing a state of being connected to an mobility analyzer).

As shown in FIG. 4, a Faraday cup ammeter (weak ammeter) 1 is a differential electric machine that classifies fine particles by utilizing the difference in the moving speed (electrical mobility) of charged fine particles in an electric field. It is connected to the mobility measuring device 2 and measures a weak current corresponding to the number concentration of fine particles (classified particles) after classification.

Here, the Faraday cup ammeter 1 includes a Faraday cup (current source) 10 for collecting fine particles and generating a weak current corresponding to the number concentration thereof, and a preamplifier for amplifying the weak current generated by the Faraday cup 10. A device (amplifier) 20 and an ammeter display unit 3 for displaying the current value of the weak current amplified by the preamplifier device 20.
It has 0 and.

Of these, the Faraday cup 10 is constructed as a double metal container consisting of an outer container 11 and an inner container 12. The outer container 11 and the inner container 12 are provided with an introduction part 13 for introducing fine particles from the outside, and the inner container 12 is equipped with a metal filter 14 for depositing the fine particles introduced from the introduction part 13. Has been. A brass rod member (first electrode) 16 for guiding a weak current to the outside of the filter 14 is connected to the lead wire 1.
It is connected via 5. The rod-shaped member 16 is the outer container 1
It is fixed by a fixing ring 17 to a plate 18 made of Teflon (tetrafluoroethylene resin) provided at the bottom of 1.

The preamplifier device 20 has an outer box 25 that is grounded and a preamplifier component 26 that is electrically isolated from the outer box 25 by an insulating member 28. Outer box 2
5 is provided with a metal fixing ring 27 that abuts the fixing ring 17 of the Faraday cup 10, and a receiving portion (second electrode) 21 connected to the preamplifier component 26 is provided at the center of the fixing ring 27 as an insulating member. It is arranged via 28. One end of the rod-shaped member 16 is connected to the receiving portion 21, and a weak current corresponding to the number concentration of fine particles deposited on the filter 14 of the Faraday cup 10 is applied to the lead wire 15,
It is adapted to be guided to the preamplifier component 26 via the rod-shaped member 16 and the receiving portion 21. The rod-shaped member 16
A metal shield ring (cylindrical member) 22 that shields electromagnetic waves incident on the connection part is provided at the connection part of the receiving part 21.

FIG. 1 is a diagram showing a connection state between the Faraday cup 10 and the preamplifier device 20.

As shown in FIG. 1, the rod-shaped member 16 is a bottom surface (first end surface) 18 of a plate 18 of the Faraday cup 10.
The receiving portion 21 is attached so as to project from a.
Is attached to. Here, a male screw 16a is engraved on the outer surface of the end portion of the rod-shaped member 16 protruding downward from the fixing ring 17, and a female screw 21a is engraved on the inner surface of the receiving portion 21, so that the rod-shaped member 16 receives the receiving portion 21. It is designed to be connected by screw fastening. The shield ring 22 is provided on the bottom surface 18 a of the plate 18 and the preamplifier device 2.
The bottom surface 18 is sandwiched between the outer case 25 and the upper surface 25a of the outer case 25.
The connection portion of the rod-shaped member 16 and the receiving portion 21 is surrounded in a state in which stress (a direction in which they are separated from each other) is applied to the a and the upper surface 25a.

As described above, according to the first embodiment of the present invention, the shield ring 22 is provided on the bottom surface 18a of the plate 18 of the Faraday cup 10 and the outer box 2 of the preamplifier device 20.
5, the rod-shaped member 16 is sandwiched between the upper surface 25a and the upper surface 25a.
Since the receiving portion 21 is connected to the receiving portion 21 by screw fastening, it is possible to effectively shield the electromagnetic wave incident on the connection portion of the rod-shaped member 16 and the receiving portion 21 through which the weak current flows, and thus the weak current to be measured. The noise appearing in can be effectively reduced.

Further, according to the first embodiment of the present invention,
Since the shield ring 22 is sandwiched between the bottom surface 18a of the plate 18 of the Faraday cup 10 and the top surface 25a of the outer box 25 of the preamplifier device 20, stress is applied to the bottom surface 18a and the top surface 20a. 1
6a and the female screw 21a of the receiving portion 21 are in good contact with each other on the screw surface, so that the noise appearing in the weak current to be measured can be effectively reduced.

In the first embodiment described above, the Faraday cup 10 and the preamplifier device 20 are electrically connected to each other by the male screw 16a of the rod member 16 and the receiving portion 2.
However, the present invention is not limited to this, and the Faraday cup 10 and the preamplifier device 20 can be electrically connected by various connection methods such as a point contact method. is there.

In the first embodiment described above, the metallic shield ring 22 is used as the cylindrical member for shielding the electromagnetic wave incident on the connecting portion between the rod-shaped member 16 and the receiving portion 21. The shape of the tubular member is not limited to the cylindrical shape, and may be any shape such as a tubular shape having a rectangular cross section, a washer shape, a spring washer shape, or the like. Further, as shown in FIG. 2, a metal bellows 23 having vertical elasticity may be used as a cylindrical member for shielding an electromagnetic wave incident on a connecting portion between the rod-shaped member 16 and the receiving portion 21. Good. When the tubular member has elasticity as described above, the contact state between the male screw 16a of the rod-shaped member 16 and the female screw 21a of the receiving portion 21 on the screw surface becomes better, and therefore, the weak current to be measured is measured. The noise appearing in can be reduced more effectively.

Further, in the above-described first embodiment, the metal ring 22 is used as the cylindrical member for shielding the electromagnetic wave incident on the connecting portion of the rod-shaped member 16 and the receiving portion 21, For the material of the tubular member,
For example, a metal such as aluminum can be used,
It is also possible to use a member having a metal layer provided on the surface of a plastic material.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. The second embodiment of the present invention is intended to reduce the noise appearing in the weak current to be measured by improving the contact state at the connection portion of the rod-shaped member and the receiving portion, and the basic configuration The above is substantially the same as that of the first embodiment shown in FIGS. 1, 2 and 4. In the second embodiment of the present invention, FIG.
The same parts as those in the first embodiment shown in FIG. 4 are designated by the same reference numerals and detailed description thereof will be omitted.

FIG. 3 is a view showing a connection state between the Faraday cup 10 and the preamplifier device 20.

As shown in FIG. 3, a rod-shaped member (first electrode)
16 is attached so as to project from the bottom surface (first end surface) 18a of the plate 18 of the Faraday cup 10, and the receiving portion (second electrode) 21 is an outer box 25 of the preamplifier device 20.
Is attached to the upper surface (second end surface) 25a. Here, a male screw 16a is engraved on the outer surface of the end portion of the rod-shaped member 16 that protrudes downward from the fixing ring 17, and the receiving portion 21
An internal thread 21a is engraved on the inner surface of the rod-shaped member 16
Is connected to the receiving portion 21 by screw fastening. Further, between the bottom surface 18a of the plate 18 and the top surface 25a of the outer box 25, a spring (cylindrical member) 24 which applies stress (in a direction away from each other) to the bottom surface 18a and the top surface 25a.
Is sandwiched between.

As described above, according to the second embodiment of the present invention, the spring 24 causes the plate 18 of the Faraday cup 10 to move.
Bottom surface 18a of the preamplifier 20 and the upper surface 2 of the outer box 25 of the preamplifier device 20
Since the bottom surface 18a and the top surface 25a are sandwiched between the male screw 5a and the upper surface 25a, stress is applied to the bottom surface 18a and the upper surface 25a.
The state of contact between the receiving portion 21 and the female screw 21a on the screw surface becomes good, and therefore noise that appears in the weak current to be measured can be effectively reduced.

In the first embodiment described above, the bottom surface 18a of the plate 18 of the Faraday cup 10 is used.
The spring 24 is used as a tubular member that applies stress to the upper surface 25a of the outer box 25 of the preamplifier device 20, but the present invention is not limited to this, and a ring-shaped member, a washer-shaped member, a spring washer-shaped member, a bellows-shaped member, or the like. Any member of can be used.

[0027]

EXAMPLES Next, specific examples of the weak ammeter shown in FIGS. 1 and 4 will be described.

FIGS. 5 (a) and 5 (b) show changes with time of the zero-point signal (current value in the state where fine particles do not flow into the Faraday cup 10) detected by the weak ammeter shown in FIGS. 5A shows the experimental results when the shield ring 22 is sandwiched between the Faraday cup 10 and the preamplifier device 20, and FIG. 5B shows the Faraday cup 10 and the preamplifier. Device 20
It is a figure which shows the experimental result when the shield ring 22 is not pinched between and. It should be noted that the zero-point signal observed in this manner reflects the influence of noise as it is.

As can be seen by comparing FIGS. 5A and 5B, when the shield ring 22 is sandwiched between the Faraday cup 10 and the preamplifier device 20, the fluctuation range of the signal is about 0.1 fA. The time transition was stable (Fig. 5 (a)). On the other hand, when the shield ring 22 is not sandwiched between the Faraday cup 10 and the preamplifier device 20, the fluctuation range of the signal becomes a level of about 2 fA, and its time transition is unstable (Fig. 5 ( See b)).

As shown in the results of this experiment, when the shield ring 22 is sandwiched between the Faraday cup 10 and the preamplifier device 20, it is more than the conventional weak ammeter.
Since it is possible to perform accurate measurement (measurement of a weak current of less than 1 fA) with a precision of the order of magnitude, it is possible to accurately measure even a weak current that corresponds to the number concentration of a very small amount of fine particles.

[0031]

As described above, according to the present invention, it is possible to effectively reduce the noise appearing in the weak current to be measured, and therefore, the weak noise corresponding to the number concentration of a very small amount of fine particles. The current can be measured precisely.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a main part of a first embodiment of a weak ammeter according to the present invention.

FIG. 2 is a cross-sectional view showing a modified example of the main part of the first embodiment of a weak ammeter according to the present invention.

FIG. 3 is a cross-sectional view showing an example of a main part of a second embodiment of a weak ammeter according to the present invention.

FIG. 4 is a sectional view showing a state in which a Faraday cup ammeter (weak ammeter) is connected to a differential type electric mobility measuring instrument.

FIG. 5 is a diagram showing an influence of noise in a weak ammeter.

[Explanation of symbols]

1 Faraday cup ammeter (weak ammeter) 2 Differential type electric mobility measuring instrument 10 Faraday cup (current source) 16 Rod-shaped member (first electrode) 18 plates 18a Bottom surface (first end surface) 20 Preamplifier device (amplifier) 21 Receiving part (second electrode) 25a Upper surface (second end surface) 22 Shield ring (cylindrical member) 23 Bellows (Cylindrical member) 24 spring (cylindrical member) 30 Ammeter display

─────────────────────────────────────────────────── ───Continued from the front page (56) References Actual flats 3-101470 (JP, U) Actual flats 2-91143 (JP, U) Actual flats 5-58560 (JP, U) (58) Field (Int.Cl. ⁷ , DB name) G01R 19/00-19/32

Claims (5)

(57) [Claims] 1. A current source that generates a weak current to be measured, and an amplifier that amplifies the weak current generated by the current source, wherein the current source is attached to the first end face and the first end face. And a rod-shaped first electrode for guiding a weak current to the outside, the amplifier is attached to the second end surface and a second end surface facing the first end surface, and the amplifier is configured to guide the weak current to the inside. A second electrode connected to the first electrode, the second electrode serving as a receiving portion that receives the rod-shaped first electrode, and the first end face of the current source and the second end face of the amplifier. A weak current ammeter characterized in that a tubular member surrounding the connection portion of the first electrode and the second electrode is sandwiched between and so as to shield an electromagnetic wave incident on the connection portion. 2. The tubular member surrounds a connection portion of the first electrode and the second electrode in a state where stress is applied to the first end face and the second end face. Weak ammeter described. 3. The weak ammeter according to claim 1 or 2, wherein the first electrode and the second electrode are connected by screw fastening. 4. The weak ammeter according to claim 1 or 2, wherein the tubular member has elasticity. 5. The weak ammeter according to claim 1, wherein the current source is a Faraday cup that generates a weak current corresponding to the number concentration of charged fine particles.