This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Where electric current is available, it can be used to heat an incubator much better and cleanlier than the kerosene lamp. The materials are inexpensive and the cost should be no more than for the ordinary kind of heater.
First of all the box part must be made of very dry wood, 1/2 in. thick. The material should be matched, as the cost of the operation depends upon the construction of the box. The proper size for an 80-egg incubator is 2 ft. square and 1 ft. high. If a larger one is desired, the dimensions may be varied to suit, but it is not necessary to make it any higher for a larger one. If it is desired to have a window in the door, care must be taken to make it a good fit. The top, as shown in the sketch, is made without hinges so that it can be readily set on and removed. This makes it handy in case of repairing the heater and cleaning the box. The inside of the box, with the exception of the bottom, should be covered with asbestos paper.
Ill: Fig. 1 - Box Details
After the box is finished, fit it with a tray, 1 1/2 ft. by 1 ft. 10 3/4 in. A tray having these dimensions will slide easily in the box. This is an essential feature of the hatching. The frame of the tray D, Fig. 1, consists of wood, 3/4 by 3/4 in., with a bottom made of wire mesh. The mesh should be firmly attached, so that it will not give away when full of eggs. Runners for the tray are placed 4 1/2 in. from the bottom of the box. When the tray is put in place, it will not touch the back. This small space is left for the chicks to fall into the nursery below. About 4 in. below the tray four holes are bored, A A, 1/8 in. in diameter, one on each side of the box. These holes admit fresh air to the eggs.
Ill: Fig. 2 - Heater Details
The electric heater is just large enough to allow a space about 1/2 in. on all edges. This makes it 23 in. square. A piece of 1/4-in. asbestos of the above size should be secured, on which to place the heating wire. The amount of wire depends on the size and kind. As it is not necessary to heat the wire very hot, iron or steel wire may be used. The length of wire may be determined by the following method:
Wind the wire on a long stick, making sure that no one coil touches its neighbor. Connect one wire of the current supply at one end of the coil and run the other end of the current supply along the coils, starting at the extreme opposite end and drawing toward the center until the iron wire gets too hot to hold with the bare hand. This will be the right length of wire to use. The length being known, a number of tacks are placed in the asbestos board to hold the wire, as shown in Fig. 2. Cover the wire with a sheet of asbestos and attach binding-posts, E and F, at each end.
The asbestos inclosing the heating wires is covered with a thin piece of sheet iron, which is made to fit tightly over the bottom and sides. This will spread the heat evenly. Be careful to have the binding-posts insulated from the sheet metal. In the cross section of the heater, Fig. 2, A represents the 1/4-in. asbestos board; B, the heater wire; C, the asbestos paper, and D the sheet-metal covering.
The most important part of the incubator is the thermostat which regulates the current to maintain a steady heat. It is not advisable to make this instrument, as a good one can be purchased for less than $1. Place the thermostat in the end of the box at B, Fig. 1. A small door, E, is made in the box for easy adjustment of the thumbscrews.
Suspend the heater from the cover of the box with bolts 2 3/4 in. long, as shown in Fig. 1. A base receptacle, G, and a snap switch, H, are fastened on top of the cover and connected up to the thermostat B, the condenser C, the heater F, and lamp I, as shown. Another snap switch, J, is used on the light only. The condenser C is to prevent sparking, thus saving the platinum points on the screws. Do not use more than a 2-cp. lamp for lighting purposes, as a brighter light blinds the young chicks.
The incubator should be run for a day or two so that the current may be well regulated before placing the eggs in the tray. The incubator is operated the same as with lamp heat. - Contributed by M. Miller, Lansing, Mich.