---
title: "History of the metric system"
chunk: 5/10
source: "https://en.wikipedia.org/wiki/History_of_the_metric_system"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:11:40.036698+00:00"
instance: "kb-cron"
---

=== The first structured metric system: CGS ===
In 1861, a committee of the British Association for the Advancement of Science (BAAS) including William Thomson (later Lord Kelvin), James Clerk Maxwell, and James Prescott Joule among its members was tasked with investigating the "Standards of Electrical Resistance". In their first report (1862), they laid the ground rules for their work—the metric system was to be used, measures of electrical energy must have the same units as measures of mechanical energy, and two sets of electromagnetic units would have to be derived—an electromagnetic system and an electrostatic system. In the second report (1863), they introduced the concept of a coherent system of units whereby units of length, mass, and time were identified as "fundamental units" (now known as base units). All other units of measure could be derived (hence derived units) from these base units. The metre, gram, and second were chosen as base units.
In 1861, before a meeting of the BAAS, Charles Bright and Latimer Clark proposed the names of ohm, volt, and farad in honour of Georg Ohm, Alessandro Volta, and Michael Faraday respectively for the practical units based on the CGS absolute system. This was supported by Thomson (Lord Kelvin). The concept of naming units of measure after noteworthy scientists was subsequently used for other units.
In 1873, another committee of the BAAS (which also included Maxwell and Thomson) tasked with "the Selection and Nomenclature of Dynamical and Electrical Units" recommended using the cgs system of units. The committee also recommended the names of "dyne" and "erg" for the cgs units of force and energy. The cgs system became the basis for scientific work for the next seventy years.
The reports recognised two centimetre–gram–second based systems for electrical units: the Electromagnetic (or absolute) system of units (EMU) and the Electrostatic system of units (ESU).

=== Electrical units ===

In the 1820s, Georg Ohm formulated Ohm's law, which can be extended to relate power to current, electric potential (voltage), and resistance. During the following decades, the realisation of a coherent system of units that incorporated the measurement of electromagnetic phenomena and Ohm's law was beset with problems—several different systems of units were devised.
In the three CGS systems, the constants 
  
    
      
        
          k
          
            e
          
        
      
    
    {\displaystyle k_{\text{e}}}
  
 and 
  
    
      
        
          k
          
            m
          
        
      
    
    {\displaystyle k_{\text{m}}}
  
 and consequently 
  
    
      
        
          ϵ
          
            0
          
        
      
    
    {\displaystyle \epsilon _{0}}
  
 and 
  
    
      
        
          μ
          
            0
          
        
      
    
    {\displaystyle \mu _{0}}
  
 were dimensionless, and thus did not require any units to define them.
The electrical units of measure did not easily fit into the coherent system of mechanical units defined by the BAAS. Using dimensional analysis, the dimensions of voltage 
  
    
      
        
          
            
              M
            
          
          
            
              1
              2
            
          
        
        
          
            
              L
            
          
          
            
              1
              2
            
          
        
        
          
            
              T
            
          
          
            −
            1
          
        
      
    
    {\displaystyle {\mathsf {M}}^{\frac {1}{2}}{\mathsf {L}}^{\frac {1}{2}}{\mathsf {T}}^{-1}}
  
 in the ESU system were identical to the dimensions of current in the EMU system, while resistance had dimensions of velocity in the EMU system, but the inverse of velocity in the ESU system.

==== Electromagnetic (absolute) system of units (EMU) ====
The Electromagnetic system of units (EMU) was developed from André-Marie Ampère's discovery in the 1820s of a relationship between currents in two conductors and the force between them now known as Ampere's law:

  
    
      
        
          
            
              F
              
                m
              
            
            L
          
        
        =
        2
        
          k
          
            m
          
        
        
          
            
              
                I
                
                  1
                
              
              
                I
                
                  2
                
              
            
            r
          
        
      
    
    {\displaystyle {\frac {F_{\text{m}}}{L}}=2k_{\text{m}}{\frac {I_{1}I_{2}}{r}}}
  
 where 
  
    
      
        
          k
          
            m
          
        
        =
        
          
            
              μ
              
                0
              
            
            
              4
              π
            
          
        
         
      
    
    {\displaystyle k_{\text{m}}={\frac {\mu _{0}}{4\pi }}\ }
  
 (SI units)
In 1833, Gauss pointed out the possibility of equating this force with its mechanical equivalent. This proposal received further support from Wilhelm Weber in 1851. In this system, current is defined by setting the magnetic force constant 
  
    
      
        
          k
          
            
              m
            
          
        
      
    
    {\displaystyle k_{\mathrm {m} }}
  
 to unity and electric potential is defined in such a way as to ensure the unit of power calculated by the relation 
  
    
      
        P
        =
        V
        I
      
    
    {\displaystyle P=VI}
  
 is an erg/second. The electromagnetic units of measure were known as the abampere, abvolt, and so on. These units were later scaled for use in the International System.

==== Electrostatic system of units (ESU) ====
The Electrostatic system of units (ESU) was based on Coulomb's quantification in 1783 of the force acting between two charged bodies. This relationship, now known as Coulomb's law, can be written

  
    
      
        
          F
          
            
              e
            
          
        
        =
        
          k
          
            e
          
        
        
          
            
              
                q
                
                  1
                
              
              
                q
                
                  2
                
              
            
            
              r
              
                2
              
            
          
        
        ,
      
    
    {\displaystyle F_{\mathrm {e} }=k_{\text{e}}{\frac {q_{1}q_{2}}{r^{2}}},}
  
 where 
  
    
      
        
          k
          
            e
          
        
        =
        
          
            1
            
              4
              π
              
                ϵ
                
                  0
                
              
            
          
        
      
    
    {\displaystyle k_{\text{e}}={\frac {1}{4\pi \epsilon _{0}}}}
  
 (SI units)
In this system, the unit for charge is defined by setting the Coulomb force constant (
  
    
      
        
          k
          
            e
          
        
      
    
    {\displaystyle k_{\text{e}}}
  
) to unity and the unit for electric potential was defined to ensure the unit of energy calculated by the relation 
  
    
      
        E
        =
        Q
        V
      
    
    {\displaystyle E=QV}
  
 is one erg. The electrostatic units of measure were the statampere, statvolt, and so on.

==== Gaussian system of units ====
The Gaussian system of units was based on Heinrich Hertz's realisation, while verifying Maxwell's equations in 1888, that the electromagnetic and electrostatic units were related by:

  
    
      
        
          c
          
            2
          
        
        =
        
          
            1
            
              
                ϵ
                
                  0
                
              
              
                μ
                
                  0
                
              
            
          
        
      
    
    {\displaystyle c^{2}={\frac {1}{\epsilon _{0}\mu _{0}}}}
  

Using this relationship, he proposed merging the EMU and the ESU systems into one system using the EMU units for magnetic quantities (subsequently named the gauss and maxwell) and ESU units elsewhere. He named this combined set of units "Gaussian units".