Prestolog Model V Profit Meter

This Profitmeter and Discount Calculator Model V  was designed, patented and fabricated by Henry Kempenich (1880-1970) in Palo Alto, California, since 1935 until 1943, when was substituted by a redesigned model. I have not found information when the Prestolog Profitmeters production ended, but I can assume that continued fabricating and commercializing during the 1940's. The Model V was fabricated with two concentric movable white celluloid disks riveted at their centers, scales printed on front face of each disk, the back face is blank, and no cursor.

 Henry Kempenich seems to be working in the design and production of Profitmeters by the late 1910's,  according with an entry of the 1919 Catalog of Copyright (pp 96) for the patent of instructions  for using a prestolog. and a prestolog profitmeter  made of 2 concentric celluloid disks 5 and 6.5 inches. These patents were given in November 1918. The brand Prestolog was registered as Class 26 Measuring and Scientific Appliances in September 1918, according with the entry in the US Patents Office Official Gazette, June 3, 1919 pp. 159. So, Henry Kempenich produced his first profitmeter in 1919, modifying the scales of this initial design years later. I have not seen physically this first model, I only found some pictures of it that are shown at the bottom of this page.

The 1935 Model V is structurally identical to the 1919 design, a circular slide rule with only three scales: an outer  scale on the 5" disk corresponding to Sale Price, and a double scale on the 4.5" disk, one for merchandise units, and the other for percents.  Brief and clear How to Use Instructions came with the rule explaining to the novice user the capabilities of the profitmeter: 

• Find the selling price of an item knowing the cost and the desired percent profit

• Find the percent profit given the item cost and the selling price

• Find the item cost given selling price and percent profit 

• Find the selling price of an item when the cost is given by the dozen, several dozens, or by gross.

• Find the percent profit when are known the cost by dozen(s) and the selling price per unit

• Calculate discounts

The 1943 version had a clear design difference respect the previous model, the divisions between the 3 Doz. and 8 Doz disappeared, the percentages scales was extended from 80% to 200%, and compressed to the 0 - 3 Doz. scale, and calculates profit on cost price, but the profitmeter capabilities are basically the same.

I have not found any reference or serial number to determine the exact fabrication year of the item in this collection, but it can be said that is about 80 years old.

I have recently found (Nov-2022)  the Model VC: the  Profitmeter for Citrus Fruit,  designed to compute perccent of selling price, and giving the computation of selling price per dozen, knowing the cost and count per case. This model has a copyrighted in 1924, and calculates the profit on selling price.   

These are the instrucitons foer the use of Model VC:

Here are some pictures of the original 1919 Prestolog Profitmeter I found browsing in the WEB: 

Apendix. Profit on Selling Price - Profit on Cost Price. What is the Difference?

The fundamental principle on which business operate is to sell good and services for a price high enough to cover all expenses and provide a reasoneable profit, and is quantified by the Basic Retailing Equation:

Selling Price = Cost + Markup  

 S = C + M

Where markup (M) is the amount adeed to the cost (C) of the item  to cover the operating expenses and have a profit. 

When the profit is calculated on the cost of the item, the markup is assumed to be a percentage (P) of the cost price:

S = C + P x C

S = (1 + P) x C

When the profit is calcualted on the sale price of the item, the markup is assumed to be a percentage (P) of the sale price (S):

S = C + P x S 

S - P x S = C

(1 - P) x S = C

S = C / (1 - P)

References: 

• Brechner, Robert A: Contemporary Mathematics for Business and Consumers. Second Edition. The Dryden Press Harcout College Publishers, 2000