Mechanical hair/harp/thread/acre planimeter c. 1899 Fr. J. Berg Stockholm Brass, steel, ivory, paper Calculates irregular areas on maps or drawings 12” x 5” x 1-1/2” Original wood case The original form of this instrument has several names including hair, harp, thread or acre planimeter. It was used to calculate the area of irregular shapes on maps or drawings. It does this by dividing the area being measured into narrow, parallel strips of equal width. The planimeter is first placed over the area to be measured. Then, the length of each parallelogram formed by the strip width and the outer edge of the area is measured using dividers and a rule. The lengths were then added together and multiplied by the strip width and a scaling factor to calculate the area. The sides of the parallelograms were fine threads or horsehair. These were attached to equally spaced holes in the frame (see photo). The instrument looked somewhat like a harpsichord hence the name harp. This style of planimeter may have been in use as early as the 1600s and as late as the 1960s. Because the threads often broke, later versions used lined transparent paper (see photo). The mechanized planimeter was the next evolution and is capable of calculating and measuring at the same time. After the lined, transparent paper is placed over the area to be calculated the planimeter is placed on the paper in the direction that allows the wheels to run at a right angle across the strips. The knife edge pointer which is attached to the carriage can now be moved up and down each strip. When the correct ivory button is depressed, it locks the carriage to the dial indicator chain drive and as the carriage is moved, the dial registers the distance. After a strip has been registered, the planimeter is rolled to the next strip to be measured. The two ivory buttons are designed so that the strip can be read from top to bottom and the next strip, bottom to top therefore avoiding the necessity of resetting the carriage after each measurement. The dial automatically calculates the total of the lengths as they are measured eliminating the need to manually count the number and tally the length of each strip. The dial indicator has two sets of numbers printed on the heavy paper dial. It is labeled “FR. J. BERG STOCKHOLM.” The dial has a complex layout with many lines at the perimeter and a small numbered brass gear in the face. The dial and brass gear can be viewed in the photos. The pointer which is a sharp blade and the sighting V are moved from one line to the next by a carriage in the center of the instrument. The carriage moves by hand on two steel shafts. When one ivory button is pushed, it locks a chain drive from the carriage to the dial indicator moving the dial needle as the carriage is moved. The dial is recording the amount of movement which equals the travel of the pointer. The chain is about 19-1/2” long and there are about 14 links per inch, making it about 273 miniature links in all! The instrument rolls on four brass wheels. One of the wheels has a small brass flap that acts as a handbrake. In practice, it would have taken a great deal of skill and patience to accurately measure the strip lengths. The instrument is in fine condition, noting one missing and one loose shipping brace that belong in the lid of the 12" x 5" x 3" original wooden case.