Method Of Making A Lamp With Oscillating Filament

Abstract

A method is disclosed for selecting the proper filament length in an incandescent lamp designed for operation from a source of alternating current of a given frequency, the lamp having a magnet producing a magnetic field which reacts with the current through the filament to cause the filament to oscillate. The length of the filament is selected in accordance with the frequency of the operating current to enhance the probability of the filament oscillating.

Description

This invention relates to incandescent lamps and more particularly to incandescent lamps having an oscillating filament.

Incandescent lamps which operate on alternating current and have a magnet either inside or outside of the lamp envelope for producing a magnetic field to react with the current flowing through the lamp filament for the purpose of making the lamp filament oscillate are known in the art. Such lamps are disclosed in U. S. Pat. Nos. 3,237,053, 3,548,255 and 3,549,946.

Lamps of the foregoing type normally use a carbon filament. Carbon filaments are well known in the art and there are many methods for producing these filaments and for aging these filaments prior to operating in the lamp. Heretofore, filaments for these lamps were made primarily on a basis such that the filament would have a length sufficient, when formed to the desired shape, so as to produce a desired visual effect when oscillating.

Because of variances from batch to batch in the manufacturing procedures for carbon filaments nominally made to the same specifications, differences are present in the final filament structures. These differences are due to small variations in the chemical composition of the filaments, for example, the percentage of binder material, impurities, etc. Other variances occur due to such factors as the pressure applied to form the filament, the time and temperature used for baking out the filament, and other factors. All of these contribute to differences in the filaments of various batches. When these filaments are used in an application where they do not have to oscillate, the differences are insignificant or meaningless.

In manufacturing lamps of the type having an oscillating carbon filament, a major problem encountered is a high number of rejects due to filaments which will not oscillate or which have a limited degree of oscillation insufficient for consumer acceptance. In a typical case, using the same manufacturing procedure (e.g. same method of filament aging, same degree of vacuum, same outgassing, etc.) and the same types of components (e.g. same field strength magnets, same envelopes) for making a number of lamps, the filaments of some lamps will oscillate while others will not. The reason for this has heretofore been unexplained.

In accordance with the subject invention, applicants have found that a direct relationship exists in manufacturing a lamp of the type having an oscillating carbon filament between the frequency of the alternating current applied to the lamp and the length of the filament, all other factors of manufacturing being equal or substantially equal. That is, from a given batch of carbon filaments having the same, or substantially the same, characteristics, which normally occurs because the filaments of the batch are all manufactured at the same time, an optimum length exists for the filaments so that the largest number of filaments of the batch will oscillate at a given frequency. Therefore, the present invention provides an incandescent lamp with an oscillating filament in which the filament is selected to have an optimum length to ensure the best probability that the largest number of filaments in the same batch will oscillate.

It is therefore an object of the present invention to provide an incandescent lamp with an oscillating filament and the method of making the same.

A further object is to provide an incandescent lamp with an oscillating filament in which the filament is selected to have a predetermined size relationship with respect to the frequency of alternating current supplied to the lamp.

An additional object is to provide an incandescent lamp having an oscillating carbon filament in which the selection of the filaments on the basis of length and other characteristics increases the productivity yield .

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings, in which:

FIG. 1 is an elevational view of a lamp made in accordance with the invention, and

FIG. 2 is a view of the lamp of FIG. 1 turned by 90.degree..

Referring to FIG. 1, there is shown an incandescent lamp which includes an envelope 10 of a suitable translucent material, such as glass. The envelope 10 is sealed by a base 12 having the usual electrical contacts 13 and 14 thereon through which alternating current from a suitable source (not shown) is supplied to the lamp. The base covers a stem 16 used for exhaust purposes. The upper portion of the stem is shown. All of the foregoing structure is conventional.

The stem 16 has a pair of lead wires 18 and 19 passing therethrough which are attached within the base to the electrical contacts of the base member. The ends of the lead wires 18, 19 extend above the stem 16 and a filament 20 is connected thereto at the mounting points 18a and 19a. This mounting can be accomplished by a suitable adhesive of conductive material. The filament 20 is preferably of carbon material which has been bent into a desired shape such as the modified loop shown. The ends of the filament 20 are mounted to the lead wires at points 18a, 19a by any suitable means, for example a conductive adhesive.

A support pin 25, which can be of conductive or insulating material, has one end embedded in the stem. A permanent magnet 30, for example of ALNICO or any other suitable ferromagnetic material or alloy thereof, is mounted at the top end of pin 25. The magnet can have any desired shape, the only requirement being that it produces adequate flux to interact with the current of the filament to cause the filament to oscillate. As shown, the longitudinal axis of the magnet is generally transverse to the plane of the loop of the filament so that the lines of flux are also transverse to the plane of the filament 20. However, this orientation is not absolutely necessary and there can be an angular orientation between the longitudinal axis of the magnet and the plane of the loop of less than 90.degree..

In operation, alternating current is supplied to the lamp of FIG. 1 and flows through the filament 20. The interaction of the lamp current and the magnetic field from magnet 30 produces a force in accordance with classical electromagnetic theory. This force causes the filament to oscillate to and fro on the mounts 18a, 19a as the direction of the current flowing through the filament alternates.

In accordance with the invention, it has been found that the length of the carbon material in the filament loop 20, that is, between the points of connection 18a and 19a to the lead wires 18 and 19, is critical with respect to the oscillation of the filament, all other conditions of manufacture and selection of components being equal or substantially equal. That is, assuming filament material of the same composition made by the same manufacturing process, the total length of the filament between the points of connection 18a, 19a with the lead wires 18, 19 will determine whether or not the filament will oscillate at a given frequency of alternating current applied to the lamp. This fact has not previously been recognized. Generally, in ordering filaments from a commercial manufacturer, a general overall length of filament, with tolerances, is specified. The filaments are manufactured in a U-shape and they are used in the lamps without any additional trimming of the filament length after the original U-shape has been modified to a predetermined loop configuration. In general, a high number of rejects have been encountered, that is, the filaments of some batches will not oscillate or will oscillate very poorly and will not be acceptable to a customer.

In accordance with the present invention, the lengths of the filament (from points 18a to 19a) are "trimmed" or selected to have a length which will be proper for the filament to oscillate at the operating frequency applied to the lamp. In practice, the length of the filaments are selected to include the portions used for mounting (that is, the inactive portion which is either attached to lead wires 18, 19 or extend beyond the lead wires) since the mounting of all filaments is carried out in the same way using the same overall length of filament material.

One way of determining the proper length is to start with a batch of carbon filaments which have been made at the same time and which have been cut roughly to the same overall length. The fact that the filaments were manufactured at the same time ensures that they should have the same, or substantially the same, characteristics. Sets of filaments (e.g. six filaments to a set) are then made up from this batch for test purposes, the difference in the filaments from one set to the next being only in length. Sets of lamps are used instead of individual lamps to reduce the possibility of error due to various causes. For example, the filaments of the different sets can differ by one millimeter, or one-half millimeter, in length.

The sets of filaments which are so trimmed are then mounted in lamps which have the same components. These lamps are processed to completion in the same way and the finished lamps are then operated from a source of alternating current of the same frequency for which the lamps are designed. From the total group of lamps, the lamps of one set will exhibit the best overall oscillating characteristics and/or have the greatest number oscillate. The filaments of this set are all of the same length.

After the optimum length of the filaments for the particular batch has been determined, the remaining filaments in the batch are cut to this optimum length. That is, the filaments of the batch are all cut so that their lengths between mounting points 18a and 19a will be the same.

It has been found that filaments obtained from the same manufacturer and made to the same specifications will differ considerably insofar as their ability to oscillate is concerned. The filaments often have to be trimmed by as much as approximately .+-.2 percent of their overall manufactured length to make the filaments ocillate.

By utilizing the principles of the subject invention, the manufacturing yield of lamps of the foregoing type has been raised by a considerable degree.

It should be understood that filaments trimmed to a given length to oscillate when operated by current of one frequency will not operate with current of a frequency which is considerably different. For example, filaments cut for oscillation at 60Hz will not operate when the current is 50Hz, or even 55Hz. Since higher frequency operating currents normally result in lamps with shorter filament lengths for successful oscillation, if lamps for use with different frequencies are being manufactured, the filaments for the lower frequency lamps should be selected first since these are longer and can be used for higher frequency operation if trimmed down too far.

While the illustrative embodiment of the invention shows magnet 30 inside of the lamp envelope, it should be understood that the magnet can be mounted outside of the envelope, provided the magnet's flux strength is sufficient to produce filament oscillation.

A rigorous mathematical treatment for the dependence of length of the filaments needed for oscillation has not been made. However, as pointed out above, it is believed that as between filaments manufactured to the same specifications but at different times, the differences in the ability to oscillate occur due to variances in chemical composition or processing factors which would be considered to be minute or even insignificant when the same filaments are used in applications where they do not have to oscillate. These variances can be overcome and the filaments made to oscillate, or have the greatest probability of oscillating, by trimming them to the appropriate lengths for the particular batch of filaments.

Claims

What is claimed is:

1. The method of producing incandescent lamps with oscillating filaments for operation from a source of electric current of a given frequency comprising the steps of

providing a number of envelopes of the same type and a number of permanent magnets having substantially the same magnetic field strength,

providing a batch of a number of filaments having substantially the same chemical and physical characteristics, including length, from having been manufactured under substantially the same manufacturing conditions,

taking at least one sample from said batch of filaments,

determining the optimum length of the sample filament taken from the batch for producing oscillation when operated at said given frequency with one of said number of magnets in a said envelope,

trimming the lengths of the other filaments of said batch to said optimum length as required, and

mounting each of said other filaments in one of said envelopes with a respective one of said number of magnets.

2. The method of claim 1 wherein said determining step includes taking a sample number of filaments from said batch of filaments, trimming the filaments of the sample number to different sizes, then operating said sample number of filaments in respective lamps at said given frequency to determine which filament length has the best oscillating characteristics.

3. The method of claim 2 further comprising the step of trimming all of the filaments of the batch to the same length as the filament having the best oscillating characteristics.