Page:Popular Science Monthly Volume 88.djvu/456

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��Popular Science Monthly

��only a few thousandths of an ampere, small flashlight batteries may be em- ployed. The selenium cells should be capable of carrying at least two or three milli-amperes without heating.

The next and probably the most deli- cate step in the entire construction is the ultra-sensitive relay that is placed in circuit with each selenium cell. These should operate reliably on a change in current strength of as little as twenty- five millionths of one ampere.

The finest of polarized relays, such as those devised for use with coherers in the early stages of wireless tele- graphy, require an operating current of at least five hun- dred microam- peres, or one- half a milli-am- pere ; the most sensitive galvan- ometer relay with solid con- tacts requires about two hun- dred microam- peres. These values are for conditions of jar and vibration such as those which naturally exist in the elec- trical dog. The relays that I use are the most sensitive of the pivoted, galvanometer type; but in- stead of having two solid contacts of plat- inum, one is made of platinum with a needle point, and the other is a globule of mercury, an arrangement which requires a very small contact pressure for reliable operation under vibration.

A drop of light oil over the mercury prevents oxidation. This contact, how- ever, cannot break currents in excess of a few milliamperes and should therefore be used in conjunction with relays of the telegraph type, which are capable of handling the currents required in the motor and solenoid circuits. Less sen- sitive instruments cannot be used unless

���Diagram of

the electrical

connections

��the source of light be very powerful. The sensitiveness of this arrangement is so high that a dog can be operated with ease from a distance of twenty feet with a pocket flashlight.

The pony relays indicated in the dia- gram are ordinary telegraph relays of twenty ohms resistance, provided with a special pair of back contacts, which are always closed when the relay is not en- ergized.

The motor is a ten-volt battery motor of the largest size obtainable (about fif- ty watts). Its source of power should be a storage battery, which also supplies the solenoids. In my apparatus this bat- tery was composed of four four-volt, thirty-ampere-hour cells. They shovild be as small and as light in weight as possible.

The solenoids are approximately five inches long and three inches in diameter, with cores three-fourths of an inch in diameter. Of the iron-clad type, they are wound with number sixteen magnet wire, and have cone-shaped pole faces, the air gap being inside the coil near the middle ; the stroke is about one-half inch from the central position. Their purpose is to turn the steering wheel.

The core, which extends from one solenoid to the other, is maintained in the central position when both the sole- noids are energized.

The single rear wheel is mounted on ball bearings in the horizontal plane to facilitate turning by the steering mag- nets.

The reversing switches, by means of which the dog can be made to back away from the light, instead of being at- tracted to it, are not shown in the dia- grams as they would introduce an un- necessary amount of complication. Their purpose is to reverse the connections of the two solenoids.

The driving motor is connected to the shaft of the two forward wheels through a worm-wheel reduction, and a differ- ential gear box, such as those on auto- mobiles.

The adjustment of the parts of the dog is sometimes a rather difficult task, particularly when other sources of illum- ination besides the flashlight are en- countered. If used in a room with win- dows through which daylight passes it

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